Is Gauge what really matters most when we are looking for the right string?
Yes, of course, Gauge is an objective measurable factor in a string. Easy measurable. And it is the thing early music brands use to label a string, so it is universally accepted as THE thing we should matter of.
I am not going to say that it is not important or that it is not objective. It certainly is. But it is hugely affected by other factors difficult to ponder.
When we diligently do our math to calculate our perfect string set, or when we just have kept a record of our strings so we know what to order, are we really sure we get the result we expect?
I am not thinking of production issues or errors in measures or labelling. So what?
Well, we all know that different materials, having different weights, require a different gauge to work at the same tension/pitch/length. A steel string is much thinner than the correspondent gut one.
But gut is gut, so it is not a matter of weight.
When we check if our strings are evenly tensed by pulling them with our finger at one cm from the bridge, what we are checking is the working string, the string already stretched. THIS gauge, not the one written in the label, is giving us the correct feeling. THIS gauge is giving us the right pitch and the sound that we like. The gauge of the stretched string.
Gut can stretch more or less, depending on the quantity of twisting received, and on the chemical treatments made to have it stiffer. So we can guess something looking at the string, trying to tip the end on our fingertip to feel the stiffness, but we will not have a precise mathematical coefficient for our calculation of the correct gauge unless we make experiments and we are keen in math as engineers are.
That's why a gauge is a consistent trustable factor to reorder our strings only if we stick to the same manufacturer. If we change manufacturer, we will probably have to adapt our gauges to his kind of string.
Moreover, the gauge has no value at all when we speak of wound strings. Not only we cannot see and value the gut core features, if there is a silk cushion between core and wire, but we cannot know, by simply measuring the external gauge, the gauge of the metal, the ratio between metal and core, which is the main component of a wound string.
This is why when we order or reorder a wound string, the gauge is not something useful to the string maker to understand what is it.
And this, in the end, is why we need to test empirically our set of strings when we change brand or order a different string.
This is the official pro technique.
The one I use for my strings is slightly different: instead of the pencil I use my teeth to keep the loop open, more practical.