When a guitar string is fretted, it's stretched slightly, which makes the note go sharp. And because each string is a different thickness, each one is stretched a different length.
The human ear can hear a slightly sharp note more readily than one that is slightly flat, so this little bit of sharpness can make a difference. One of the ways that this is dealt with is by fudging the scale length. The 12th fret is the mid-point of the scale length, so, theoretically the front of the saddle should be the same distance from the 12th fret as the 12th fret is from the nut. However, most luthiers move the saddle just a smidge farther away from the 12th fret--about 1/16" or .062" on the treble or first string side, and 5/32" or .156" on the bass or sixth string side. Lengthening a string makes it go slightly flat, so this compensates for the stretching of the fretted string.
However, we can dial it in even more precisely--and this is the part that the big manufacturers just can't afford to take the time to do.
I loosen the strings enough to slide a small section of the B string onto the saddle, then tune the guitar up. With a digital tuner, I compare the open (unfretted) low E string with the same string fretted at the 12th fret, which is also an E one octave higher. If the fretted E matches the open E perfectly, I don't have to do anything. If it's sharp, then I move the piece of B string back, farther away from the fretboard. If it's flat, I move the wire towards the fretboard. I move the wire around until the open and fretted notes are the same, and mark this position on the saddle with a pencil.
After doing this with all six strings, this is what the saddle looks like:
As you can see, the 2nd string (counting from the left) needs a good bit of compensation. You can take that to the bank: the 2nd string will always need significant compensation. This is the hardest string to tune, and on a guitar with an uncompensated saddle you'll often see the player fiddling with the B string. You're just never satisfied. (I'm jumping ahead, but I can testify that on this guitar with the 2nd string properly compensated, the open B sings with all sorts of overtones. It's the best open B I've ever heard on any guitar.) The fifth and sixth strings need to be pushed back a bit too, while the 1st, 3rd and 4th strings are pretty much on pitch. This is also pretty typical.
With the saddle marked I loosen the strings and remove them from the bridge. I remove the saddle and file the front of the saddle so that the break point is at the place marked for each string. Then I file the back of the saddle so that it is rounded up to the break point. This will eliminate the possibility of the string buzzing on a flat section of the saddle--pretty rare but still a possibility. I don't want the break point to be a sharp, pointed edge because this will cause a lot of string breakage, especially on the wound strings where a sharp edge can force its way between the windings. So I round the break point so that each string sits nicely on a smooth edge.
All that's left for the saddle is to sand it up to 600 grit like the nut to achieve a polished but not plastic look.
Guess what? This guitar is almost done! All that's left to do is to make and install a truss rod cover, give the frets and the body a final polish so they really shine, and apply some lemon oil to the ebony bridge and fretboard to clean and polish them a bit. All this is much easier to do without the strings installed, so I have to wait to see how it really sounds with a finished saddle.
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