Did you know that the six strings of the guitar do not have all the same tension?

Did you know that the six strings of the guitar do not have all the same tension?

Contrary to what one might think, the strings of a classical guitar set do not follow an equal tension profile.

Tension evaluation method:

The most common method used to evaluate the degree of tension of a string on a guitar, is to apply a pression, using the right hand fingers, right near to the bridge: a set is considered well balanced if all the strings oppose the same force to the pressing finger. One might come to the conclusion that all strings have been calculated with the same working tension.

But this kind of evaluation is actually a ‘tactile sensation’ of the tension, and not the real tension expressed in kilograms or pounds, as one could measure using appropriate instruments. Following a law of Physics, two strings that have the same lateral movement by means of the same pressure applied on the same point (as could be the pressing finger), will also have the same tension, expressed in Kg (or lb). But this same tension does not correspond with the one used in the necessary calculation to determine the gauges of the string.

The physical nature of the strings:

When they are exposed to traction, the strings stretch (this is particularly more evident when turning the peg during the tuning); this implies, as a consequence, a progressive thinning of the diameter. But together with the reduction of the diameter, there will also be a reduction of the working tension, proportional to the originally calculated tension. The stretch amount is not the same for all strings; its maximum is observed on the chanterelles, it will be less on the second, and even less on the third: it is well known that the number of turns on the peg of the chantarelle is way higher than the ones needed for the third string. As a consequence, a set that has been calculated using an equal tension, once tuned correctly would be completely unbalanced.
This fact is inevitable: a guitar not only has strings of heterogeneous physical nature (the first three are synthetic monofilaments, while the basses, instead, are composed of two paired materials, like a synthetic core and a metallic wounding), but its diameters are different and each string is used at a different “Working Index” (that is expressed as the product of the frequency and the vibating length of the string). As a result of all this, the achievement of a homogeneous tactile feel of the tension among the strings is actually a more complex thing as compared to the simple theoretical calculation, where the parameter for the tension is assumed as a constant.


The scaling of the tension is therefore a compensation process that is carefully studied and applied by the string-maker in order to neutralize, string by string, the thinning of the diameter caused by the stretching of the string during traction: once stabilized in their tuning, each string will decrease its value in percentage until they reach a working tension similar to the other strings. Thus all the strings will show the same bending, when pressed with a finger. From a practical point of view, if the chanterelle will experimentally decrease of a 2%, the diameter in the starting calculations needs to be increased by a 2%, and so on with all the remaining strings.

But, in practice, the gradient of the tension profile needs to be even more emphasised: the aim of this exaggeration of the scaling of the tension is to more efficiently oppose the increase of frequency of the thicker strings on the higher frets (on the second string and even more on the third). In the past, when gut strings were used, this exaggeration was not needed because thicker strings were automatically produced using a higher torsion (making them more elastic), while with modern synthetic monofilaments each string has the same stretching coeffficient.
This measure for synthetic strings, however, has been proven not to be completely enough; a series of compensatory measures on the bone of the bridge is therefore often needed, such as changing its inclination.

Choosing the tension: light, normal or superior?

There are no scientific criteria that can predict what is the best tension for our instrument. It would be like guessing how many spoons of sugar a stranger might want in his coffee. It is indeed a personal choice. In the process of choosing the optimal tension, there are several factors at play, like the type and model of the instrument, the personal sensibility, the touch and the hand of the performer. Generally speaking, if there is no previous experience, it is advised to start with a normal tension setting. Once the strings are installed and stabilized, it is possible to try and raise or lower the intonation of a half-tone, in order to understand if a Superior or Light tension could better satisfy the player’s necessities. All our Normal tension sets are within the mean values of Normal tensions produced by other string makers (very similar values in Kg per string)

I wanted to give you the external diameter of a wound string of my instrument thinking that it would be useful data for the calculation of the new string I need, but I was told that this data is useless: is this true?

Yes. The measurement of the outer diameter is only useful in strings made of only one material, such as those made of pure gut, Nylon, Nylgut, or metal only. Wound strings are composite strings, i.e. made of different materials combined together.

A wound string is characterised by these two parameters:

  • its ‘Gut Equivalent’
  • its ‘Metallicity Index’

Gut Equivalent

Since wound strings are the combination of heterogeneous materials, it has been agreed to characterise them in terms of equivalent gut: in practice, this refers to the diameter of a theoretical gut string with the same weight per unit length as the wound string under consideration. At the same pitch and vibrating length, both strings will have the same working tension. And this data is useful for calculating the required diameters.

How can we obtain the gut equivalent of a wound string that needs to be replaced and of which we know nothing?

Answer: the string must be weighed, using a precision scale that measures grams and decimals, and then the entire length is measured. The weight, expressed in grams, is then divided by the length expressed in meters. The square root of the resulting number is the ‘gut equivalent’ (expressed in mm).

Example: my wound string weighs 35.5 grams and it’s 98 cm long.

Therefore: 35.5 g / 0.98 m = 36.22 (whose square root is…) = 6,02 mm

In practice, the wound string under examination is equivalent to a theoretical gut string of approx. 6 mm in diameter.

Metallicity Index

For a given gut equivalent, a wound string can be made with innumerable ratios of metal / gut percentages.

It is obvious that as one component increases, the other decreases, in order to keep the total weight of the string, i.e. its gut equivalent, constant.

The greater the prevalence of gut over metal, the more opaque the sound will tend to be. The opposite is true if there is more metal. The correct ratio between the metal percentage to the gut core percentage is dictated purely by aesthetic taste, which refers to that particular timbral/dynamic mixture one has in mind, and which is subjectively judged ‘beautiful’.

In other words, there is no real formula that can tell us what proportion must be used. The intention here is to state that, once the value of a string’s equivalent gut (in other words, its correct working tension) has been established, the balance between metal and core can only be obtained through experience. The Metallicity Index is also related to the position occupied by the string within the instrument. In other words, the third wound string of the cello must have a lower Metallicity Index than the fourth. The latter string must in fact have a greater prevalence of the winding metal than the core (increase in Metallicity Index) in order to compensate for the natural loss of brightness due to the lower Acoustic Quality Index value (see this faq).

Two strings with the same equivalent gut value may well have completely different Metallicity Indices.

This is the case, for example, with the fourth string of the viola and the third string 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. That’s why a G of a cello can’t be installed as a C of a viola: the working tension might be right, but the acoustic performance would be decidedly unsatisfactory.