Key (music)

In music theory, the term key is used in many different and sometimes contradictory ways. A common use is to speak of music as being "in" a specific key, such as in the key of C major or in the key of F-sharp. Sometimes the terms "major" or "minor" are appended, as in the key of A minor or in the key of B-flat major. Although the concept of musical key can be a complicated subject when examined closely, broadly speaking the phrase in key of C means that C is music's harmonic center or tonic. Note that the letter-name "C" does not indicate a single specific pitch but rather all pitches with the letter name C (sometimes called a pitch class). The terms "major" and "minor" further imply the use of a major scale or a minor scale. Thus the phrase in the key of E major implies a piece of tonal music harmonically centered on the note E and making use of a major scale whose first note, or tonic, is E. Although the term "key" is commonly used this way, actual music can rarely be described so simply. This overview of the term also makes many assumptions and may not hold true for all forms of music.

A key relationship is the relationship between keys, measured by common tones and nearness on the circle of fifths. See: closely related key.

Keys and tonality

Although many musicians confuse key with scale, a scale is an ordered set of notes typically used in a key, while the key is the center of gravity, established by particular chord progressions.

The chords used within a key are generally drawn from the major or minor scale associated with the tonic triad, but may also include borrowed chords, altered chords, secondary dominants, and the like. All of these chords, however, are used in conventional patterns which serve to establish the primacy of the tonic triad.

Cadences are particularly important in the establishment of key. Even cadences which do not include the tonic triad, such as half cadences and deceptive cadences, serve to establish key because those chord sequences imply a unique diatonic context.

Short pieces may stay in a single key throughout. A typical pattern for a simple song might be as follows: a phrase ends with a cadence on the tonic, a second phrase ends with a half cadence, then a final, longer, phrase ends with an authentic cadence on the tonic.

More elaborate pieces may establish the main key, then modulate to another key, or a series of keys, then back to the original key. In the Baroque it was common to repeat an entire phrase of music, called a ritornello, in each key once it was established. In Classical sonata form, the second key was typically marked with a contrasting theme. Another key may be treated as a temporary tonic, called tonicization.

In common practice period compositions, and most of the Western popular music of the 20th century, pieces always begin and end in the same key, even if (as in some Romantic-era music) the key is deliberately left ambiguous at first. Some arrangements of popular songs, however, will shift up a half-step or a whole step sometime during the song (often in a repeat of the final chorus) and thus will end in a different key. This is an example of modulation.

Instruments in a key

Similarly, some instruments may be said to be built in a certain key. For example, a brass instrument built in B-flat will play a fundamental note of B-flat, and will be able to play notes in the harmonic series starting on B-flat without using valves, fingerholes, or slides or otherwise altering the length of the vibrating column of air. An instrument built in a certain key will often, but not always, have its music written in the same key (see trombone for an exception). However, some instruments, such as the diatonic harmonica and the harp, are in fact designed to play in only one key at a time: accidentals are difficult or impossible to play.

In Western musical composition, the key of a song has important ramifications for its composition:

Key coloration

Historical irregular musical temperaments usually have the narrowest fifths between the diatonic notes ("naturals") producing purer thirds, and wider fifths among the chromatic notes ("sharps and flats"). Each key then has a slightly different intonation, hence different keys have distinct characters. Such "key coloration" was an essential part of much eighteenth- and nineteenth-century music and was described in treatises of the period.

For example, in tunings with a wolf-fifth, the key on the lowest note of the fifth will have a dramatically different sound than the other keys (and is often avoided). In Pythagorean tuning on C (C, E+, G: 4, 5, 6), the major triad on C is just while the major triad on E#+++ (F) is noticeably out of tune (E#+++, A+, C: 4.125, 5, 6) due to E#+++ (521.44 cents) being a Pythagorean comma(23.46 cents) larger sharp compared to F.

Modern music lacks key coloration because it uses equal temperament in which all keys have the same pattern of intonation, differing only in pitch.

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