Relative scale

Two keys are relative when they share exactly the same notes, in the same order, but with a different root: the relative minor scale of a major key starts on that major scale's 6th degree, a minor third below. C major (C-D-E-F-G-A-B) and A minor (A-B-C-D-E-F-G) are therefore the exact same seven notes: only the starting point changes.

The improvisation trick

This relationship enables a widely used trick: improvising over a major-key progression using the minor pentatonic scale of its relative minor, without changing a single note compared to the major scale itself — only the feeling of "rest" changes, depending on whether phrases land on the major root or on its relative minor.

On Guitarshelf

Several improvisation sheets in the catalog apply this trick directly, for example a C major pop progression paired with the A minor pentatonic, its relative.