Despite the name, which sounds like one of those horrible foot infections, this term really refers to something quite pleasant, at least when correctly set. One of the common ‘problems’ we are presented is ‘my guitar doesn’t stay in tune’. This seems simple enough, but as usual, it’s not. We have to clarify whether the player means that after a string is tuned its pitch changes, OR when a string is in tune, the fretted notes are not. The first case refers to tuning stability, while the second refers to inaccurate intonation. Totally different procedures are employed in correcting these issues. This article deals with the concept of intonation and how to set it on a guitar. Most guitarists are familiar with the term harmonics. When a string is touched lightly above the 12th fret and plucked, it produces a note that is exactly one octave above the fundamental or open string note. If the same string is now pressed at the 12th fret and the note played, it should sound exactly the same pitch as the harmonic. If it does, then intonation at the 12th fret is correct. Assuming the frets are correctly positioned, (not always the case!), the rest of the fretted notes should also be true. What can be done if the intonation is not correct? Here we come to the design of the guitar. Electric guitars have adjustable saddles in general, which are designed to allow fine setting of action and intonation. A string can be slightly shortened or lengthened by moving the saddle towards or away from the neck. A range of a few millimetres is all that is needed when the bridge is in the right position. But what happens on a classical or an acoustic which only has a single saddle for all 6 strings? These types of saddle are only about 3 millimetres thick at the most. We can file the top to move the peak slightly closer to or further from the neck, to improve intonation. At best, this is a compromise, but better than nothing. A number of factors affect intonation, namely string length, string gauge, action, string quality and condition, and finger pressure. The last point is something over which the player has immediate and direct control. Pressing harder will raise the pitch slightly. An accurate electronic tuner, is usually sufficient to check and set the intonation. If a fretted note shows flat, the string needs to be shortened, if sharp, lengthened. Once correctly set, the guitar sounds sweet and harmonious – guaranteed to bring a smile to the player!