Fretboard Straight Edges
I'm sure you have read articles about adjusting your guitar's truss rod or your guitar's action. The number one important part about both of these setup repairs is the straightness of your guitar neck. The straightness of your guitar neck not only dictates the height of the strings, it also contributes to proper intonation and playability. It is very important to straighten your guitar neck before you any other repairs on the frets or setup. It is especially important before you fret a guitar. That is what the fretboard straight edge is made for.
If you are refretting a guitar, you need to make sure the fretboard is straight before you install the frets. Sometimes straightening the truss rod does not fully straighten the fretboard. Occasionally you have to level the fretboard. The fretboard straight edge is a standard machinist's straight edge. It can be anywhere from 16 inches all the way up to 24 inches for bass guitars. A straight edge is basically a large ruler that is precision ground on one side until it is perfectly flat and straight. To tell if your fretboard is straight, all you have to do is place the fretboard straight edge on your fretboard and shine a light behind it. If your fretboard is bowed or warped, you will see light shining through the gap in between the straight edge and the fretboard. You will either need to straighten the truss rod or level the fretboard.
A fretboard straight can only really be used on top of a fretless fretboard. If the guitar has frets on it, the straight edge will just pick up the inconsistencies in the fretónot the fretboard. If you want to measure the straightness of a fretboard with frets, you will need a notched straight edge.
Both of these straight edges are good tools to have. It is important to straighten the guitar neck before you do any setup or fret repair work. I recommend buying each of these straight edges. You will be prepared to straighten any guitar that comes across your guitar repair bench. If you would like more information about how to straighten a guitar neck, please see my how to set your guitar action article.