Citation:
On guitars, there are two popular fretboard inlay schemes:
* The most popular (1) involves single inlays on the 3rd, 5th, 7th, 9th, double inlays on the 12th, single inlays on the 15th, 17th, 19th, and 21st, and if present, double inlays on the 24th.
Advantages of such scheme include its symmetry about the 12th fret and symmetry of every half (0-12 and 12-24) about the 7th and 19th frets. However, playing these frets, for example, on the E string would yield the notes E, G, A, B, C# that barely make a complete musical mode by themselves.
* A less popular scheme (2) involves inlays on 3rd, 5th, 7th, 10th, 12th, 15th, 17th, 19th, 22nd and 24th frets. Playing these frets on the E string yields the notes E, G, A, B, D
that fit perfectly into the E minor pentatonic scale. Such a scheme is very close to the coloring of a piano's keys (which typically use black coloring for the sharp notes of a pentatonic scale) and is of some use on classical guitars.