In music theory, the key of a piece is the group of pitches, or scale, that forms the basis of a musical composition in Western classical music, art music, and pop music.

Tonality (from "Tonic") or key: Music which uses the notes of a particular scale is said to be "in the key of" that scale or in the tonality of that scale.

A particular key features a tonic note and its corresponding chords, also called a tonic or tonic chord, which provides a subjective sense of arrival and rest, and also has a unique relationship to the other pitches of the same key, their corresponding chords, and pitches and chords outside the key. Notes and chords other than the tonic in a piece create varying degrees of tension, resolved when the tonic note or chord returns.

The key may be in the major or minor mode, though musicians assume major when this is not specified; for example "This piece is in C" implies that the key of the piece is C major. Popular songs and classical music from the common practice period are usually in one key. Longer pieces in the classical repertoire may have sections in contrasting keys. Key changes within a section or movement are known as modulation.