We know this thanks both to historical documentation rediscovered in recent years and to presumably antique string fragments.

The color of gut strings has no influence whatsoever on either their acoustic performance or their lifespan. It is merely a natural pigmentation that varies from one gut to another. However, excessively white strings (similar to Nylon) might indicate a bleaching treatment that was too aggressive.

No, the gut of any animal, if unbleached, will always appear more or less brown.
In other words, this simply means that that particular type of string (in this case, mutton gut) – by a deliberate choice of the string maker – has not undergone a material bleaching treatment.

If the strings are all made from split gut (strips), nothing changes. The acoustic quality of a string essentially depends on two factors: the density of the material and its elastic modulus.
Historical value aside, all these materials possess the same density (after all, it is dried meat).
The elastic modulus, on the other hand, depends exclusively on how the string maker crafted the string (high or low twist; use of hardening salts, etc.).
Double-blind tests conducted so far have shown that it is impossible to perceive any substantial acoustic difference.
Things change radically, however, if the strings are made from whole lamb gut rather than cut into strips. Here, we actually find a better acoustic yield, faster intonation, and greater string stability.
In any case, there are records indicating that until the mid-17th century, the guts of goat, lamb, wether, mutton, and sheep (and more rarely wolf and beef) were used interchangeably in various regions of Italy (see Athanasius Kircher: ‘Musurgia Universalis’, Rome 1650).

Today (as of 2019), this statement is no longer true. Cutting gut into strips to make musical strings is a technique already used from the second half of the 16th century (see the Statutes of the String Makers of Rome, 1589, and Naples, 1653) and, even more certainly, in the second half of the 18th century, but in Italy it was always considered a commercial fraud.
Until recently, the manufacturing process of whole gut was no longer known—meaning in its finest details. Generally speaking, it should also be noted how difficult it is today to find lamb guts thin enough to, for example, achieve—by twisting three of them together—a diameter equal to the average of a historical violin treble string (cantino): in practice, you always end up with much thicker strings, a sure sign that it is not true lamb (under 1 year of age). Hence the necessity to split it in half to produce thinner strips, as was done outside Italy and then also in this country starting from 1910-20.
After several decades of research, our company has finally succeeded in making strings from whole gut and no longer from strips.

There can be several causes:

you are not yet accustomed to playing with heavier gauges than usual.
thicker strings produce excessive pressure on the soundboard, which suffocates the sound produced: decrease the pressure by reducing the string angle over the bridge, using a shim placed between the tailpiece and the edge of the soundboard (a solution also adopted by the ancients), or, if possible, lower the bridge itself.
the strings are too stiff due to the type of raw material, the type of chemical phases adopted, and the degree of twist.
For further details, we suggest visiting this website: www.damianstrings.com/baroque%20set-up.htm

An important difference: a high twist string is essentially much more elastic, which will consequently react easily both when plucked and under the bow, producing a great dynamic range and timbral variety.
Conversely, a low twist string – which is stiffer – is less reactive but has greater tensile strength: it is therefore intended for treble strings in general.
For the same material, elasticity and tensile strength are inversely proportional: this is why a high twist string cannot be used as a treble; it would break easily.
It works perfectly, however, if made with a low twist. Finally, if the latter were to be used in middle registers, the result would not be as good: the acoustic performance would be impoverished.

Today, a modern string is polished using a special machine until a perfectly smooth surface is achieved: this certainly comes at the expense of the surface fibers, which are thus inevitably damaged.
In a semi-rectified string, however, the polishing operation is stopped before a completely smooth surface is reached: in this way, we not only manage to best imitate the manual polishing of the past,
but we also achieve the result that most of the surface fibers remain substantially intact.
This translates into a longer string life, better tuning stability, and improved acoustic yield (broken fibers actually produce a dampening effect on the sound).

Generally none. For double bass strings, however, it is suggested to treat them occasionally, in the section subject to the action of the left-hand fingers, with deer fat available in pharmacies. This will increase their lifespan.
If you cannot find deer fat, feel free to use colorless hiking boot grease: it has been tested with complete success and is certainly preferable to that of a living creature.

Download this video in webm format or in mp4 format.

None in particular. Moisten them along their length with olive or almond oil if, in particularly dry and cold climates, a slight buzzing of the wire against the inner core occurs.

You must perform the simple test described by Ganassi, Mersenne, Le Roy, and others, which allows you to quickly distinguish a false string from a true one. In practice, you stretch the string between your hands and repeatedly pluck it with a finger about 5 cm from one end, carefully observing the quality of the vibration envelope. The envelope must be uniform without any wavering inside. This test was widely performed because string polishing was done freehand, with the consequent risk of imprecision in the final diameter. Nowadays, thanks to mechanical rectification, it is generally superfluous.

The Breaking Index of a string expresses the maximum high frequency at which the string precisely breaks, for any diameter (as proven by the string formula) at a unitary vibrating length of 1 meter. For gut and Nylgut, the average value of 260 Hz · m is considered valid (in other words, this means that at a vibrating length of 1.0 meters, a gut – or Nylgut – string will statistically break at a frequency of 260 Hz: roughly a ‘C’).
It is therefore evident that the product of the treble string’s intonation frequency and the instrument’s vibrating length (more properly called the ‘Working Index’) must therefore always be below a safety margin, otherwise resulting in immediate string breakage (if the value of 260 is exceeded) or breakage within a short period if you are between an index of 240 and 260. No problems below 230. This applies to plucked instruments; for bowed instruments, the safety value is 220 Hz · m, intended as a maximum value.
What is it for??
This formula is useful for checking whether a given vibrating length is excessive or not for the intonation required for the treble string. It is therefore invaluable for luthiers.
Example: is it possible to tune a lute with a 62 cm vibrating length to A (a = 440 Hz pitch)?
Answer: 62 cm = 0.62 meters. Thus: 0.62 m X 440 Hz (frequency of the treble A) = 272.8
The answer is no.
What should be the correct vibrating length to tune a lute to A?
For the safety of the treble string, we operate with an Index less than or equal to 240.
Therefore: 240 / 440 Hz (treble frequency) = 0.545 meters.
In practice, a vibrating length of no more than 54 cm should be adopted.
Rule of thumb for plucked instruments:

Working Index less than or equal to 230: green light (the treble string is in a sufficiently safe condition)
Working Index between 240 and 250: orange light (the treble string could statistically break within a few hours/days, especially in high humidity conditions)
Working Index greater than 260: red light (the treble string breaks instantly or after a few minutes).
For bowed instruments:

Working Index less than or equal to 200: green light (the treble string is in a sufficiently safe condition)
Working Index between 210 and 220: orange light (the treble string could statistically break within a few hours/days, especially in high humidity conditions)
Working Index greater than 240: red light (the treble string breaks instantly or after a few minutes).
Double Basses and Violones: do not exceed a Working Index of 190.
Due to the low degree of standardization, this check is particularly necessary for medieval instruments and their respective harps, or even for exact copies of museum instruments, lutes above all (it is by no means certain that the vibrating length of the original instrument – specifically optimized for the patron’s pitch at the time – is also suitable for our modern, generally higher, pitch!).

It is the maximum diameter that is still capable of producing an acoustic yield that is generally acceptable to our ears (in other words, its degree of inharmonicity). The limit is empirical and primarily a function of the string’s degree of elasticity. Then follow the external factors, namely:

the vibrating length, whether the instrument is plucked or bowed
the quality of the instrument
the working tension.
Here too, it can be expressed as an Acoustic Quality Index, equal to the product of the instrument’s vibrating length in meters and the frequency in Hz of the string in question.
Empirically speaking, in the family of mid-sized lutes in G (built nowadays with a 60-62 cm vibrating length), a 1.0 mm string (5th course) made with high twist begins to be so dull in itself that it requires an octave pairing. The 1.40 mm diameter, in the case of a cable-woven string (Venice type), can undoubtedly be considered the extreme limit of acceptability (the 6th course of a lute). In the case of a bowed instrument of equal vibrating length, this limit is empirically raised by a factor of 1.1.
Why do we need to know this?
It serves to decide, when calculating diameters, when it is time to switch from a natural gut, Nylgut, or Nylon string to a wound one.
Rule of thumb:

Acoustic Quality Index of 100: a gut/Nylgut/Nylon string begins to show a certain loss of acoustic quality (necessitating, for example, octave strings in Lutes)
Acoustic Quality Index less than 80: it is necessary to switch to a wound or loaded gut string.

Nowadays, the diameter is measured using a high-precision micrometer.
However, care must be taken to accurately check the zero before each measurement and not to squeeze the string too much during measurement. In both cases, skewed data would be obtained.
And in the past??
From around 1830 until the mid-20th century, musicians used a string gauge (the first description is found in LOUIS SPOHR; Violinschule […], Wien 1832).
Louis Spohr’s gauge is a metal plate with an acute ‘V’ shape with gauges scored on the edges: the string slid in until it touched both sides of the V. At this point we read its gauge value.
This method, obviously, allows for a certain margin of approximation. We have no record of similar methods being used in the 18th century.
Marin Mersenne (Harmonie universelle, Paris 1636) suggests winding the string around a small cylinder a given number of times tightly together, measuring its entire width and dividing by the number of spires.

The scaling of diameters (e.g., 62, 64, 66 etc.) is a relatively modern invention; adopted after the introduction of precision mechanical rectification.
Up until the early 20th century, a string’s diameter was mainly determined by the number of guts needed to manufacture it. A violin E string, for example, generally used three.
This means that, since gut is a natural product that cannot be completely standardized in its physical dimensions, the result was not a single gauge but a range of diameters produced.
Again in the case of the violin, the range of gauges obtainable with three lamb guts varied between 0.65 mm and 0.75 mm, with a strong quantitative prevalence of intermediate gauges (0.68 mm – 0.70 mm).
Strings were generally sold in oiled envelopes containing 30-50 strings all made from the same number of guts: it was primarily up to the musician to select, using Mersenne’s test, the false strings from the good ones and then, using the string gauge, to separate those that were too thin or too thick for them.

A string is varnished in order to delay surface wear.
The varnishing operation is not historical; the first samples of varnished strings we have observed date back only to the 1920s-30s. A varnished string has a slightly opaque acoustic yield, while the attack of the bow is sometimes more difficult and prone to emitting ‘whistles’ (squeaks).

Most string breakages are attributable to the cutting effect induced by the nut and the bridge. In the late 17th century, for example (Thomas Mace, 1676), the following is suggested:
‘take a Knife, and make a little Impression upon the Nut, which must afterwards be Filed reasonable deep, to hold the Strings… after you have marked the places of all your Strings, which you may do with a Pencil, or Pen and Ink. …you must take it [the nut] and Polish it very well (but especially the Notches)… take a piece of New Neats-leather, and a little scraped Chalk, wet in Spittle, and rub it hard, till the Notches be very smooth…’
Every sharp edge and/or abrupt angle must therefore be carefully eliminated.
Only at the end of this operation do we suggest placing some lubricating graphite from a pencil or very dry soap in the grooves: this will not only result in better stability and ease of tuning, but it will also prevent the string – being soft – from flattening and jamming inside the groove, thus facilitating its possible breakage.
John Dowland, for example (“Other necessary observations belonging to the lute”, in ROBERT DOWLAND: Varietie of lute-lessons […], Thomas Adams, London 1610, paragraph “Of setting the right sizes of string”) suggested the following:
‘…the best remedy when strings stick is to rub the grooves in the nut (where the strings run) with a little oil, wax, or black lead’.

No, gut is an extremely hygroscopic material: moisture absorption therefore affects the final diameter more or less: strings measured, for example, on a rather humid day will show a larger gauge and vice versa.
It has been experimentally observed that an .82 mm gut string during a humid day can measure up to .84 mm; on the other hand, .80 mm during a particularly dry day. The diameters of our strings refer to a temperature of 20° C and 60% relative humidity.
This variation in gauge, however, has a minimal impact on the working tension.

The main enemy of gut strings is humidity: preserve the strings in nylon bags or airtight boxes.
A well-stored string retains its qualities for many decades.

Historical wound strings, lacking the compensation cushion between the core and the covering wire, can encounter this type of problem during particularly harsh and dry climates. The methods of the time (see FRIEDRICH DOTZAUER: “Methode de violoncelle”, Richault, Paris c. 1830) suggest lightly moistening the string along its entire length with a few drops of olive or almond oil in order to slightly swell the core and eliminate the buzzing problem. Never use water.

Essentially by ensuring that there are no points in contact with the string that produce a cutting effect (nut, bridge, tailpiece; see this faq). The amount of string wound around the peg should be as little as possible, avoiding too many overlaps (which make tuning stability fleeting) and avoiding it being crushed laterally against the pegbox (see THOMAS MACE: Musik’s monument […], the author & John Carr, London 1676).
Lutes: we suggest reading an interesting paper by the luthier Jiri Cepelak: http://lute.cepelak.cz/care.pdf

It is a number obtained from the product of the vibrating length in meters and the intonation frequency in Hz of the treble string. It indicates the suitability of the ‘amount’ of chosen vibrating length with respect to the required intonation; a suitability that translates on the one hand into the risk of premature breakage of the first string (if this length is excessive: see faq #14); on the other hand into the probability of acoustic quality problems arising for the lower strings (if the vibrating length is too short, a more muffled sound is obtained).
For plucked instruments like the lute and for the violin, it is good for the Working Index to remain within a range of 230-240 Hz · m. In this way, the treble string works near the breaking point, ensuring the best possible acoustic yield for the bass registers.

Nylgut is a synthetic material discovered and patented by us in 1997: it has the same average specific weight as gut and a reduced moisture absorption – equal to only 10% – of that of musical Nylon. In practice, it can be defined as a ‘synthetic gut’. Theoretically, a gut string and a Nylgut string should have the same gauge. However, due to the high longitudinal stretch of Nylgut, it is suggested to use a slightly larger diameter for the latter. Pull Nylgut strings well and repeatedly during the initial tuning phase. For the cutting effect, follow the same suggestions indicated for gut strings in this faq.
Due to attack slipping problems, Nylgut cannot be used on bowed instruments.

The topic is certainly quite vast. We therefore suggest this in-depth study.
In extreme summary:
Violin: first three strings in plain gut; fourth string wound with round wire.
The use of a modern wound third string is not historically correct. However, in 18th-century France alone, a demi-filé (open-wound) D string was in use, later replaced in the 19th century by a natural gut string.
Viola: First two strings in natural gut; third and fourth wound.
Cello: for repertoires prior to c. 1730: first three in natural gut; fourth wound.
After c. 1730: third and fourth wound.
Three-string Double Bass: the first two in natural gut; the third either in natural gut or wound.
Four-string Double Bass: the first two in natural gut; the third either in natural gut or wound; the fourth wound.
It is emphasized here how the use of modern wound strings (flat metal wire; rather stiff gut cores; silk between the core and the metal winding; balance between core and metal different from historical criteria, etc.) so widespread today heavily distorts the true expressiveness of a historically set-up instrument.

Wound strings made according to historical methods have the following general characteristics:

round section metal wire
medium or high twist gut cores
absence of silk between the core and the metal winding
use of metals such as silver, silver-plated copper, pure copper, and its alloys (brass)
characteristic balance between core and winding
Modern wound strings:

flat type metal wire
stiff and sparsely twisted gut core
presence of silk between the core and the metal winding
use of more recently discovered metals (Tungsten, Nickel, special alloys, etc.)
balance between core and metal favoring the metal
The acoustic differences are therefore notable; and affect both dynamic and timbral quality. The attack under the bow is equally different: closer to gut in the case of historical strings.

There were three types:

close-wound strings: the metal wire is wound only once around the core and the coils are placed close to each other. These are the commonly used wound strings.
double overlaid wound strings: the metal wire is wound in close coils and overlapped twice on itself. Due to the considerable amount of metal compared to the gut, their use was indicated for all those instruments that had a short vibrating length compared to the required stringing (e.g., Violoncello da spalla, 5th of the Double Bass, etc.).
open-wound strings: the metal wire is wound only once with the coils not close together but spaced out by the diameter of the wire itself or slightly more (see F. Le Cocq, Paris 1724); strings used exclusively in the 18th century as transitional strings between the upper gut strings and the close-wound basses (for example, as the 4th of the Bass viol, as the third of the Violin, etc.).
Therefore, many of the demi-filé strings available today cannot be considered historical as they are made with very open spacing and/or with the metal wire sunk into the gut.

The concept of tension expressed in Kg is an exquisitely modern concept, appearing probably for the first time around 1860 (MAUGIN – MAIGNE: Nouveau manuel complet du luthier).
Until the mid-19th century, the tension of a string was understood exclusively in terms of the tactile sensation of stiffness, which is a different thing from tension as generally understood today.
In fact, while the tension value is expressed solely by its value in Kg, the tactile sensation of stiffness is instead influenced by various parameters such as the vibrating length (this is why the strings of a Theorbo, if all diligently calculated in equal tension, the unstopped ones then feel so slack compared to those on the fingerboard!), the type of string, and its diameter. A string set where all strings are calculated at an equal tension – but with different positions and diameters within the instrument, for example – will manifest a rather different tactile sensation from one another: this contradicts the criteria indicated in the 16th and 17th-century treatises, which prescribe that a setup is well made when the strings all manifest the same homogeneous feel under the fingers. In conclusion, for the same type of string, an equal feel setup must always be of a somewhat scaled type. Click here for further details.

Assuming that the contact points with the string have been well polished and rounded, the first thing to do during the initial tuning phase is to pull the string with your hands until it remains stable in intonation: moderately for trebles and wound strings, but with more energy in the case of thicker strings.
This advice applies to both gut and synthetic strings; Nylgut especially.

It is better to follow the advice of Thomas Robinson (early 17th century): slightly lower the pitch of the treble string, thereby reducing the tensile stress.

The difference is substantial: research in the field of past string setups and ancient fragments has in fact made it possible to ascertain that, generally speaking, string gauges used were not only thicker than today’s custom, but also had hand-polished surfaces (and therefore not completely smooth).
Now, the choice to use a setup consisting of historical diameters can sometimes involve a readjustment of the instrument’s set-up (with particular reference to the angle formed by the strings at the bridge), which is sometimes not directly accepted by the instrument. This is why, alongside the proposal of a historical setup (which we always hope for), we still offer a customary type setup (in light, medium, and heavy versions).

The search for the right working tension of an instrument is essentially an empirical data point, tied to the type of instrument, the type of strings, and the musician’s sensitivity.
In other words, there are no specific mathematical formulas.
We therefore suggest the following procedure: mount on the instrument (whether plucked or bowed) a second or third string of a known diameter that you theoretically consider suitable. Then bring the string up to pitch and see if it feels (according to your own sensitivity) too slack or too tight. At this point, lower (or raise) the pitch by as many semitones as necessary so that the string is brought to the right tension value; that is to say, according to your subjective taste: meaning neither too slack nor too tight.
Now apply the following formula:
String that was originally too tight: multiply (as many times as the number of semitones you had to lower the pitch) the starting diameter by the value 0.944: every time you perform the multiplication, there will be a reduction of one semitone in diameter.
String that was originally too slack: multiply (as many times as the number of semitones you had to raise the pitch) the string’s diameter by the value 1.059: every time you perform the multiplication, there will be a growth of one semitone in diameter.
Request new strings by rounding the value determined by the calculation to the immediately closest commercial diameter.
Example: I mounted on my lute (or bowed instrument in general) a fourth string with a diameter of 82, but, once tuned, the tension is too light.
Answer: starting from the string set at the correct pitch, raise it one semitone at a time until the tension feels subjectively right.
Let’s say a two-semitone raise is needed: multiply the starting diameter (82 mm) twice by the coefficient 1.059: the correct diameter will be 91.9 mm, or a commercial 91.

Strings can whistle for various causes, even concurrent ones.
Let’s look at only the most frequent one now: a common reason why the bow’s attack produces an initial whistle may be the fact that the oil with which the strings are treated has not been carefully removed.
Therefore, before mounting them on the instrument, it is advisable to clean them thoroughly with a cloth moistened with a little spot-removing fluid (naphtha).

A gut string can snap immediately or after a few hours due to three conditions: the string is defective, the vibrating length is excessive (the Breaking Index has been exceeded), or the contact points with the string (grooves at the nut and bridge; tailpiece holes, etc.) are sharp and not lubricated with graphite or solid soap as suggested by the ancients.

Defective string: generally, a defective string is recognized because it does not usually snap cleanly but rather through progressive fraying, heralded by the appearance of small hairs and flaking along its surface.
Excessive vibrating length: check if the product of the vibrating length (in meters) and the frequency is greater than the value 240 in lutes, baroque guitars, rebecs, vielles, and medieval music instruments in general.
Breakage caused by cuts on the string: this type of breakage is characterized by an instantaneous failure of the string, which breaks at the place where the cutting effect occurs.
Attention: the string breaks while in a state of traction: this means that once it has been cut, for example at the nut, the breaking point on the string will no longer correspond to the nut because the string is no longer stretched. The breaking point will therefore appear at the second/third position or – if the bridge caused the cut – between the bridge and the tailpiece.

Venice type strings have a rather high elasticity, much higher than what can be obtained from a normal high twist string. Since elasticity and tensile strength are inversely proportional, it is clear that this type of string must not be used in positions where the Acoustic Quality Index (which is the product of the vibrating length in meters and the frequency of the string in Hz) exceeds the value of 140 Hz · m (that is to say, that of trebles in general).

DE type strings were specifically designed to fill the void that otherwise exists in the field of basses found in the short and medium unstopped strings of archlutes in general and ‘swan-neck’ d-minor lutes.
DEs are strings purposely unbalanced in favor of the Nylgut core rather than the metal covering.
Therefore, an acoustic result has been obtained that decisively tempers the typical sonority of traditional wound strings, which is dominated by acoustic persistence, an over-richness of overtones, and a poverty of the fundamental. A yield, in other words, that is excessively guitar-like.
Conversely, if mounted on lutes without unstopped strings, the acoustic yield will be centered mainly on the fundamental, making the use of paired octaves indispensable. Just like in the past, after all.
The special balance of DEs has been specifically studied in order to reconstruct the typical sonority of our loaded gut strings.

Two strings of the same diameter and made of the same material (under conditions of equal vibrating length and intonation), according to the string formula, also manifest the same working tension. This is true, however, provided that the diameter remains the same for both, even in a state of tension.
In practice, this only occurs if we are dealing with strings made in exactly the same, identical manner. In other words, they must manifest – for the same Kg – the same longitudinal stretch and, consequently, the same percentage reduction in diameter (evidenced, for example, by the same number of turns provided to the peg during tuning).
A Venice type string, by its nature, stretches much more than an ordinary high twist one. Consequently, for the same intonation frequency, its diameter will reduce proportionally more.
Hence the necessity to use a slightly thicker starting gauge: under tension, the new gauge configuration will then equal that of the ordinary string.
The working tension will therefore finally be equal.
In practice, the correct diameter for a Venice string is obtained by multiplying the gauge of a given high twist string to be replaced by a coefficient of 1.07.

No: examination of historical documentation has so far amply demonstrated that both the six-double-course guitar (late 18th/early 19th century Spain) and the single-course guitar exclusively used wound basses on a silk core.
Silk, in guitar basses, remained in use until the commercial appearance of multifilament nylon, that is around 1946-50.

Two strings of the same diameter and made of the same material (under conditions of equal vibrating length and intonation), according to the string formula, also manifest the same working tension. This is true, however, provided that the diameter remains the same for both, even in a state of tension.
In practice, this only occurs if we are dealing with strings made in exactly the same, identical manner. In other words, they must manifest – for the same Kg – the same longitudinal stretch and, consequently, the same percentage reduction in diameter (evidenced, for example, by the same number of turns provided to the peg during tuning).
A Venice type string, by its nature, stretches much more than an ordinary high twist one. Consequently, for the same intonation frequency, its diameter will reduce proportionally more.
Hence the necessity to use a slightly thicker starting gauge: under tension, the new gauge configuration will then equal that of the ordinary string.
The working tension will therefore finally be equal.
In practice, the correct diameter for a Venice string is obtained by multiplying the gauge of a given high twist string to be replaced by a coefficient of 1.07.

No, the documentation found so far (with particular reference to some methods by Sor, Carulli, and Aguado) shows that the guitar of that time used the same average gauges as the contemporary violin; in practice, these are working tensions comparable to those of today’s classical guitars.
The only exception is the setups for Neapolitan guitars from the first decades of the 19th century (Fabbricatore, for example): some documents show the use of slightly lighter gauges than those of the violin.
Furthermore, the tension profile among the six strings was decidedly less scaled than that currently followed in all commercial setups, bordering on equal tension strictly speaking.
We suggest always thoroughly checking the glue state of the bridge beforehand if you intend to fit an original guitar with a string setup.

This is a frequently encountered problem and is generally not attributable to the strings.
The main cause is due to the fact that the instrument probably has an excessively short vibrating length for the required intonation and, especially, in relation to the fact that an all-gut setup is desired.
Before the spread of wound strings – which appeared after the mid-17th century – the Viola da gamba family featured rather considerable vibrating lengths compared to those traditionally in use today.
From Marin Mersenne’s tables, for example, it is calculated that the Bass viol in D possessed a vibrating length of about 85 cm against the typical 68-70 cm of a current bass viol. The same goes in proportion for the Tenor in G: 55-56 cm currently against the 60-62 cm of the late 16th/first half of the 17th century.
Instruments of the time had a longer vibrating length in order to make the trebles work near their breaking point (Working Index no less than 220 Hz · m: see this faq). Only by means of this artifice could the minimum diameter of all strings, especially the basses, be guaranteed.
Vibrating length and diameter are in fact inversely proportional.
A reduction in the diameter of a string (at equal tension, of course) always brings a beneficial effect both in the quality of the sound produced and in the ease of attack under the bow. Exactly what the basses, the most critical strings par excellence, need.
Conversely, if the vibrating length is shorter, the strings will take on ranges of larger diameters, worsening the acoustic yield due to the increased internal damping coefficient of the string (‘inharmonicity’ index).
The solutions? Essentially three:

Use instruments with vibrating lengths typical of the period prior to the appearance of wound strings; namely 82-85 cm for the Bass; approx. 61 cm for the Tenor and approx. 41 cm for the Treble for the relative proportions between the instruments (see Thomas Mace: Musik’s monument […], the author & John Carr, London 1676). In this way, a pure gut setup will sound excellent and responsive.
If possible (in the sense that the bridge height allows it), move the bridge towards the base of the ‘C’ bouts: by doing so, the vibrating length is increased to the benefit of the overall acoustic yield; remember to calculate the diameters taking this modification into account.
Use wound bass strings (this only in case a more suitable instrument is not available or you do not intend to exploit the trick – which is, moreover, historical – of repositioning the bridge lower).

A gut string that has been sharply bent shows a whitish stretch at the point of the bend. This effect is due to a slight loss of cohesion between the fibers but does not at all indicate that the string is damaged. In other words, the string is still perfectly intact.

Try knotting the strings as the ancients sometimes did:
Unfortunately, there is no exact indication on how to proceed: it is therefore necessary to act experimentally, string by string. Not to be applied to wound strings!

Yes. Measuring the outer diameter is only useful for strings made of a single material, such as plain gut, Nylon, Nylgut, or solid metal. Wound strings are composite strings, meaning they are made of different materials coupled together.
A wound string is instead characterized by these two parameters:

its ‘Equivalent Gut’
its ‘Metallicity Index’
Equivalent Gut
Since spun strings are formed by coupling materials of heterogeneous nature, it was agreed to characterize them in terms of equivalent gut: in practice, this refers to the diameter of a theoretical gut string that has the same weight as the spun string in question, per unit of length. At the same intonation and vibrating length, there will therefore be the same working tension. And this data is useful for calculating the required diameters.
How do you find the equivalent gut of a wound string to be replaced when we know nothing about it?
Answer: weigh the string with a scale that measures grams and then measure its entire length. At this point, divide the weight in grams by the length in meters. Find the square root of the resulting number: this is the equivalent gut (expressed in mm).
Example: my spun string weighs 35.5 grams and is 98 cm long.
So: 35.5 g / 0.98 m = 36.22 (now take the square root of this value…) = 6.05 mm.
In practice, the spun string in question is equivalent to a theoretical gut string of 6 mm in diameter.
Metallicity Index
At equal equivalent gut, a spun string can be made with countless ratios between the percentage of metal and that of the gut.
Obviously, as one increases, the other decreases, in order to keep the total weight of the string constant, i.e., its equivalent gut.
The greater the prevalence of gut over metal, the more opaque the sonority will tend to be. The exact opposite in case there is more metal. The correct ratio between the percentage of metal compared to the percentage of the core is purely dictated by aesthetic taste, which refers to that particular timbral/dynamic blend you have in mind and subjectively judge as ‘beautiful’.
In other words, there is no formula that can tell us what the right degree of balance is. By this we mean that, once the equivalent gut value of a string is established (in other words, its correct working tension), the balance between metal and core can be derived exclusively through experience. The Metallicity Index is also linked to the position occupied by the string within the instrument. It is clear, in other words, that the third wound string of the cello must have a lower Metallicity Index than the fourth. This last string, in fact, must possess a greater percentage prevalence of the covering metal compared to the core (increase of the Metallicity Index) in order to compensate for the natural loss of brilliance due to the lower value of the Acoustic Quality Index (see this faq).
Two strings characterized by the same value expressed in equivalent gut can easily possess completely different Metallicity Indexes.
This is the case, for example, of the fourth string of the viola da braccio and the third of the cello. In the first case – the C of the viola – there will be a clear prevalence of metal (higher Metallicity Index) than in the G of the cello. This is why you cannot mount a cello G as a viola C: the working tension might even be right, but with a decidedly unsatisfactory acoustic yield.

Probably due to these reasons:

because the original order was subsequently modified or integrated
because strings were requested for a special instrument or one whose gauges were unknown
the address was incomplete, incorrect, or missing the tax code (mandatory since January 2007)
you didn’t add a telephone number (mandatory for courier or express parcels in general)
we were temporarily out of stock of some gauges
you were put on a waiting list because we were momentarily clogged with orders
due to a postal disservice (happens on average twice a month)

These strings were made by us based on the historical knowledge we acquired after years of research. Due to the spacing between the coils of the metal wire, this type of string is extremely delicate. During the initial tuning phase, it is therefore advisable to keep the string raised a few mm above the groove of the nut and the bridge, and let it go only when you are close to the required intonation. The spacing of the winding, in fact, does not inherently allow the string to slide within the grooves. In this way, the risk of damaging the metal wire is eliminated.