Modern tuning machines offer guitar players exceptional pitch control, but don’t forget about the weather.

When guitarists talk about their favorite instruments, the conversation often drifts toward a particular guitar that always seems to stay in tune, as if that one particular guitar possessed some magic power that always righted itself. While some recent guitar makers have attempted to do just that, the vast majority of our instruments require us to adjust the strings into harmonic alignment before we play. The degree to which we can control the exact pitch of the strings, the stability of pitch once we get them in tune, and the ease with which we make adjustments have steadily improved over the last century.

As with the violin family, the earliest version of tuning adjustment was achieved with a tapered or slightly wedge-shaped peg the string was wrapped around. In principle, this method is effective, albeit primitive. The string tension was adjusted, and the peg held in place while simultaneously being wedged more deeply into its hole—hopefully sticking in place long enough to perform before slipping loose.

The widespread adoption of the worm-gear design we currently enjoy suggests it arrived as a revelation in accuracy and ease. In addition to more predictable adjustment, one of the significant accuracy improvements came with the allowance of a gear ratio. In other words, by using two interlocked gears with unequal numbers of teeth, a tuning machine can impart a degree of independence in how many turns a player rotates the adjustment knob, and the resulting rotation of the post the string is wrapped around.

While early geared tuners had fairly close ratios, modern versions offer a wonderful degree of accuracy. As an example, a common contemporary gear ratio will be around 15:1. (This translates into 15 complete revolutions of the adjustment knob resulting in one complete rotation of the string post.) If a string needs a tuning adjustment of one-quarter rotation of the knob, the string post will respond by turning a scant 1/60th of a rotation—a mere six degrees. If we estimate the diameter of a string post as 1/4", we can calculate the circumference to be near 3/4". Once the string is snugged up around the post, the 3/4" length represents the linear amount of travel the string experiences as it is tensioned. In this context, six degrees of post rotation equates to a minimal 13/1000s of an inch of linear travel. (That’s about half the thickness of a medium guitar pick.) This extraordinarily fine adjustment means that today’s players can adjust their tuning to a level of accuracy only dreamed of in eras past.

From this practical reality comes the guitarist’s understanding that we always tune up to a pitch, never down.

Since the pitch of a string is not linearly related to its tension, but rather the square root of its tension, we can’t specify exactly how much pitch change this tiny movement produces. But it’s fair to say the net result is a high degree of control on the part of the musician.

Like all machines, these tuning gears are not without inaccuracy. Despite the precise machining used to produce them, there is inevitably a minuscule amount of mismatch and resulting space between each tooth—known in the engineering world as “backlash.” This is the non-functioning “dead” space between the interacting gear teeth. If the gear is being tightened and the teeth are pressing against each other in one direction, you’ll feel that it’s having a direct effect. If you reverse the direction, however, the driving gear needs to rotate a small amount before the opposite face of the teeth contact each other and spin in the new direction.

From this practical reality comes the guitarists’ understanding that we always tune up to a pitch, never down. While tuning up to a pitch ensures the two gears’ teeth are set against each other— preventing them from slowly loosening or unraveling—tuning down to the correct note will encourage the gears to continue rotating in response to the string’s tension, slowly creeping out of tune.

Apart from the stability of the tuning machines, another significant factor in a guitar’s pitch stability is an environment in which the guitar is mostly comfortable. While much has been said about the importance of consistent humidity and temperature for a wooden guitar, one very practical piece of evidence is how close the guitar maintains its tuning between playing sessions. So long as the strings are stretched in, firmly anchored, and not slipping, drifting out of tune is largely the result of wood subtly responding to a change in the weather. Even a few thousandths of an inch of wood expansion or contraction will shift a guitar slightly out of tune. A well-made guitar that is kept in a consistent environment, with minimal changes in temperature and humidity, will tend to stay in close tune.

Every time we pick up our favorite guitar, it feels like a familiar friend yet a new day for most of us. And that favorite guitar will have responded to whatever change blew in with the weather, whether it was a big or small departure from the previous day. The good news is that thanks to the luxury of modern and precise tuning machines, accurate musical pitch is at our fingertips.