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In music theory, the key of a piece is the tonic note and chord which gives a subjective sense of arrival and rest. Other notes and chords in the piece create varying degrees of tension, resolved when the tonic note and/or chord returns. The key may be major or minor, 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, about 1650–1900. Longer pieces in the classical repertoire may have sections in contrasting keys. The methods by which the key is established for a particular piece are not easy to explain, as they vary considerably over the period of music history; however, the chords most often used in a piece in a particular key are those containing the notes in the corresponding scale, and conventional progressions of these chords, particularly cadences, serve to orient the listener around the tonic. The key signature is not 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 will be 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 meaning it is a transposing instrument. 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 usually identifies the tonic note and/or chord: the note and/or major or minor triad that 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, and/or melodic motion (such as movement from the leading-tone to the tonic). 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. In languages other than English, other 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.〔Willi Apel, ''Harvard Dictionary of Music'' (Cambridge: Harvard University Press, 1969), 450.〕 The notes and 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 notes and chords, however, are used in conventional patterns that serve to establish the primacy of the tonic note and triad. Cadences are particularly important in the establishment of key. Even cadences that do not include the tonic note or 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. In rock and popular music some pieces, "tend to float back and forth between two keys", with examples including Fleetwood Mac's "Dreams" and The Rolling Stones' "Under My Thumb". "This phenomenon occurs when a feature that allows multiple interpretations of key (usually a diatonic set as pitch source) is accompanied by other, more precise evidence in support of each possible interpretation (such as the use of one note as the root of the initiating harmony and persistent use of another note as pitch of melodic resolution and root of the final harmony of each phrase)."〔Ken Stephenson, ''What to Listen for in Rock: A Stylistic Analysis'' (New Haven: Yale University Press, 2002), 48. ISBN 978-0-300-09239-4.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Key (music)」の詳細全文を読む スポンサード リンク
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