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In music theory, a theoretical key or impossible key is a key and scale that exists in theory and practice, but whose corresponding key signature makes it impractical to notate. The key signature for such a key would contain one or more double-flats or double-sharps. Double-flats and double-sharps indicate accidentals, but are never placed in the key signature (in music that uses equal temperament), due to notational convention, and because that would make reading the music difficult. For example, the key of is not conventionally used in notated music, because its corresponding key signature would contain a B (submediant). An equal-tempered scale of D minor comprises the same notes as the scale. Under equal temperament the scales sound exactly the same; such key pairs are said to be enharmonically equivalent. So the theoretical key of D minor is usually practically notated by a key signature of C minor. The other option is to use either no key signature or one with single-flats and to provide accidentals as needed for the Bs. ==Enharmonic equivalence== While a piece of Western music generally has a home key, a passage within it may modulate to another key, which is usually closely related to the home key (in the Baroque and early Classical eras), that is, close to the original around the circle of fifths. When the key is near the top of the circle (a key signature of zero or few accidentals), the notation of both keys is straightforward. But if the home key is near the bottom of the circle (a key signature of many accidentals), and particularly if the new key is on the opposite side (in the late Classical and Romantic eras), it becomes necessary to consider enharmonic equivalence. In each of the bottom three places on the circle of fifths the two enharmonic equivalents ''can'' be notated and so do not classify as 'theoretical keys': 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「Theoretical key」の詳細全文を読む スポンサード リンク
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