One of the most frustrating problems that many guitarists experience is getting open chords to play in tune on their instruments. How many players have had their guitars properly set up, with the action, intonation, truss rod, and string height at first fret all carefully adjusted only to find that open chords sound terrible? I certainly have. The instinct when this happens is to start messing about with the intonation at the bridge, which will make no difference to the issue at hand, and will only create new problems elsewhere on the fretboard.
The problem is with the nut. It’s in the wrong place, usually too far from the first fret, meaning that all notes at the first three or four frets will be sharp; and the reason that messing around at the bridge won’t help is that any adjustment you make there will have virtually no effect below the fourth fret.
This situation arises quite a bit, and it happens because when some guitar makers calibrate and cut the fretboard, they will cut it for the nut at the zero point of the scale length (the scale length being the distance from the front edge of the nut to the centre of the bridge). And there’s where the problem lies. The nut shouldn’t be at the zero point of the scale length, it should sit slightly short of it. So, some compensation is needed, just as it’s needed at the bridge to adjust for the fact that the strings are stretched whenever you push them down to the frets. In effect, intonation is set at both ends of the strings.
The solution is to move the nut to the correct position, although, as it turns out, the correct position can vary from guitar to guitar. This might seem like you’re changing the scale length, but that’s not quite how it works. The scale length is chosen by the makers (25.4 inches for Martin, 24.75 inches for Gibson, etc.) and from this measurement the fretboard is calibrated using the 17.817 rule. This is where the scale length is divided by 17.817 to give the position of the first fret, the remainder is divided by 17.817 to give the position of the second fret, and so on. Any compensation which is then made to the nut end of the fretboard is needed because, even when the strings are as low as possible at the first fret, the act of pushing the strings down on to the fret is enough to make them play sharp.
I’ve developed a method for nut compensation that has worked with every guitar I’ve tried it on, and I’ll go through it now. The first thing that’s needed is a sufficiently accurate tuner. Pedal tuners aren’t really suitable, and any tuner apps I’ve tried just aren’t accurate enough. In my opinion, this tuner:
the Korg GT-120, is the best digital tuner out there. The needle is smooth and accurate, and the tuner is responsive whether you’re using the built in mic, a clip on one, or a cable for electric or electro-acoustic guitars.
Using the tuner, let’s have a look at the problem at hand. Here’s the reading for the in tune open G string of a Sigma guitar I worked on.
The needle is dead centre, and both the red pointers are on. Now, here’s how it was at the first fret, G#.
This is a long process; sand back the end of the fretboard a bit, replace the nut (without glue at this stage), and check the intonation at the first fret. Repeat till the first three frets are in tune on all strings. When replacing the nut, the nut slot will obviously be wider than before, so I build the nut up at the back using a mixture of superglue and baking soda, like this: