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 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 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
The key identifies the tonic triad, the chord, major or minor, which represents the final point of rest for a piece, or the focal point of a section. Although the key of a piece may be named in the title (e.g. Symphony in C), or inferred from the key signature, the establishment of key is brought about via functional harmony, a sequence of chords leading to one or more cadences. A key may be major or minor; music can be described as being in the Dorian mode, or Phrygian, et cetera, and is thus usually considered to be in a specific mode rather than a key. When a particular key is not being described in the English language, different key naming systems may be used.
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.
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