Related key
A key closely related to the home key of a piece, such as the relative or dominant key.
What it means
Related keys are the keys closely related to the home key of a piece. They share many notes and chords and are easy to reach. They include the relative key, the dominant key, the subdominant key and their relative keys.
In C major these are A minor, G major, E minor, F major and D minor.
Musical use
Composers move to related keys to create contrast without losing coherence. In sonata form the second theme stands in a related key, and in rondos and suites the episodes move to related keys. Baroque and Classical music mostly moves within this circle.
A related key should be distinguished from the home key, in which a piece begins and ends, and from distant keys, which have little in common with the home key and are usually reached only by detours or cadences.
Performance practice
When playing, notice when a piece moves to a related key. You hear such places through new accidentals; let them sound out and give weight to the return to the home key.