Key (music)

In Western tonal music, a key represents the most common pitches and the center of tonal stability in a song or other composition.

A key has two components: a tonic pitch and a mode. The tonic pitch is represented by a letter from A through G, sometimes modified by the accidental symbols ♯ (sharp) and ♭ (flat). This tonic represents the musical pitch which a piece will be oriented around and almost always conclude with. The mode may be Major or Minor; if no mode is specified, Major is usually implied. This mode represents a pattern of ascending or descending pitches, which can create a major or minor musical scale beginning with the tonic pitch. Music in a given key will use the pitches from this scale (called diatonic pitches) more often than the pitches outside it (chromatic pitches). Together, these result in keys with names like C Major, A Minor, and B♭ Major. Not all music has a well-defined key.

Written music typically begins with a key signature. This tells the performer which pitches are diatonic and should be expected most often. However, the key signature does not specify the tonic pitch. Music can be converted from one key into another by raising or lowering all pitches, which is called transposition. Some instruments, like the clarinet and trumpet, are called transposing instruments because they usually require this; music written for these instruments will be offset from what the performer actually plays. Since the same musical patterns are found across different keys, musical chords are often expressed as numbers based on where their lowest pitches fall in a given scale. This allows common chord patterns to be easily transposed and analyzed independently from a song's key.

Relationships between keys can be visualized with a diagram called the Circle of Fifths. This diagram shows which keys share the majority of pitches with each other and which have minimal overlap. It also shows the relationships between major keys and their relative minors (which contains the same pitches but a different tonic pitch), and the pitch differences between keys with the same tonic but different modes. When two keys are closely related, it is easier to transition between them—a process called modulation.