<P> In music theory, the key of a piece is the group of pitches, or scale, that forms the basis of a music composition in classical, Western art, and Western pop music . </P> <P> The group 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 group, their corresponding chords, and pitches and chords outside the group . Notes and chords other than the tonic in a piece create varying degrees of tension, resolved when the tonic note or chord returns . </P> <P> The key may be in the major or minor mode, though musicians assume major in a statement like, "This piece is in C ." Popular songs are usually in a key, and so is classical music during the common practice period, around 1650--1900 . Longer pieces in the classical repertoire may have sections in contrasting keys . </P> <P> Methods that establish the key for a particular piece can be complicated to explain and vary over music history . However, the chords most often used in a piece in a particular key are those that contain the notes in the corresponding scale, and conventional progressions of these chords, particularly cadences, orient the listener around the tonic . </P>

What is the key of a piece of music