From time to time I’ll have conversations with players about intonation issues. Often, they’re worried about weird tuning inconsistencies that have crept in to their guitar or bass. Maybe, they’ll even have done the 12th-fret intonation check and found that it seemed ok but the problems persisted.
Assuming that the instrument’s setup is otherwise sound, one good place to start looking is at the frets themselves.
Worn frets can bring their own intonation and tuning problems. Think of it this way:
A nicely crowned fret will have a clean, rounded top, with its apex running along the middle of the fret. See the side-view in the image above.
When you finger a string at the fret, the string ‘bears’ off a point exactly in the middle (exactly over the slot in the fingerboard if you like).
That’s what it’s supposed to do. The fret slots are cut in the the exact places to minimise intonation problems.
However, when the fret begins to wear, the point at which the string bears off moves closer to the bridge. It’s not much but you don’t need much to introduce problems. Add to this, the fact that fret wear isn’t typically even all over the neck and you’ve got weird inconsistencies as you move around.
So, if you take your guitar in for intonation issues, don’t be surprised if your tech examines the frets.
The answer is some fret work — in many cases a fret-level and re-crowning but, sometimes, a refret. Either job should be followed by a good setup. The end result should be a guitar that plays as in-tune as a guitar possibly can.