In music theory, the key of a piece is a group of pitches, or scale upon which a music composition is created in classical, Western art, and Western pop music. The group features a tonic note and its corresponding chords, also called a tonic or tonic chord, providing 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. The key may be in the major or minor mode, 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, around 1650–1900. Longer pieces in the classical repertoire may have sections in contrasting keys.
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.
The key signature is not always a reliable guide to the key of a written piece. It does not discriminate between a major key and its relative minor; the piece may modulate to a different key; if the modulation is brief, it may not involve a change of key signature, being indicated instead with accidentals. Occasionally, a piece in a mode such as Mixolydian or Dorian is written with a major or minor key signature appropriate to the tonic, and accidentals throughout the piece.
Pieces in modes not corresponding to major or minor keys may sometimes be referred to as being in the key of the tonic. A piece using some other type of harmony, resolving e.g. to A, might be described as “in A” to indicate that A is the tonal center of the piece.
An instrument may be said to be “in a key”, an unrelated usage referring to the pitches considered “natural” for that instrument. For example, modern trumpets are usually in the key of B♭, since the notes produced without the use of valves correspond to the harmonic series whose fundamental pitch is B♭. (Such instruments are said to be transposing when their written notes differ from concert pitch.)
A key relationship is the relationship between keys, measured by common tones and nearness on the circle of fifths. See closely related key.

Source: Key (music) – Wikipedia