What is meant by scale length?

Scale length is defined as the actual vibrating length of the strings, on a guitar this is the measure between the fretboard side of the top nut and the saddles of the bridge. The scale length affects playability as a shorter scale length allows for a shorter neck best suited for fast position changes or players with a smaller hand span. With regard to tone, a longer scale eg tele and strat style guitars (25.5", 648mm scale length) favours brightness and cleaner overtones and more separated harmonics versus a shorter scale (e.g. Les Paul with 24.75 in (629 mm) scale length) which favours "warmth" or more muddy overtones.

This is an important metric to consider when determining the vertical positioning for your bridge. Incorrect bridge positioning will result in an inability to correctly set the intonation.

How do I measure scale length?

The scale length is denoted by 2 times the distance from the fingerboard side of the top nut to the 12th fret, or the distance from the inside of the top nut to the bridge saddle.