In music theory, the key of a piece is the tonic note and chord that provides a subjective sense of arrival and rest. Other notes and chords in the piece create varying degrees of tension, resolved when the tonic note or chord returns. The key may be major or minor, although major is assumed in a phrase like "this piece is in C." Popular songs are usually in a key, and so is classical music during the common practice period, about 1650–1900. Longer pieces in the classical repertoire may have sections in contrasting keys.