There’s an old joke that says: “Guitarists spend 90% of their time tuning, and the other 10% playing out of tune”. There’s certainly some truth in this because, as much as we love guitars, there’s nothing can drive us to near insanity like trying to get them in tune.
Most of the tuning problems that guitarists encounter stem from two sources: 1) a lack of understanding of the equal temperament tuning system that the vast majority of western music employs, and 2) flaws in a guitar’s construction. I’m going to tackle the first of these in a fairly simple way in this post, and deal with the second in Part II.
Equal temperament tuning is the system which has been used since about 1750 whereby, for instruments of fixed pitch (like piano and guitar, where the smallest interval you have is a semitone), the octave is divided into 12 equal semitones. This means that, when playing chords, some notes sound slightly out of tune (some more than others) but the amount of out of tune-ness is the same for all chords in all keys.
Think of it this way: on a guitar (and a piano), F# and Gb are the same note, but strictly speaking, they’re not. There is a gap between them (an example of a phenomenon called the Pythagorean Comma), but equal temperament tugs at the ends of both notes and pulls them together. This is why, for example, the F# note on the second fret of the high E string sounds perfectly in tune as the root of a first position F# chord, but sounds quite sharp as the major 3rd of an open D chord. The same note will also sound very slightly flat as the 5th of a first position B minor chord. In fact, for all of these chords to sound perfectly in tune, each F# note would have to be slightly different, and the same also goes for all other notes. Achieving this would mean that a guitar would need to have about 30 frets to the octave rather than 12, which is not really a practical proposition.
So, what we’ve ended up with is a tuning system where, in all chords in all keys, 5ths are 1.96 cents flat of the pure pythagorean intervals, 4ths are 1.96 cents sharp, and Major 3rds are 13.69 cents sharp of the pure interval. That’s more than an eighth of a semitone sharp (a cent being one hundredth of a semitone), which is why those of us who are sensitive to tuning struggle with them. In fact, in equal temperament tuning, the only intervals which are (or should be) perfectly in tune are octaves and unisons. This table shows the difference between the pure or “just” intervals and the tempered ones.
This is why tuning to a chord doesn’t work. The tendency will be to tune the chord to pure, rather than tempered, intervals, which will make all other chords sound terrible. If you’re tuning by ear, only unisons and octaves should be used. The thing to do is to get used to how chords sound in equal temperament; to train your ear and learn to expect that Major 3rds are going to sound sharp and 5ths flat, and so on. Do this, and you’ll be much happier with your guitar.
All this, of course, depends on your guitar being set up properly, and any flaws in its construction being dealt with, which I’ll talk about in The Joys and Pains of Tuning Part II.