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In musical notation, a key signature is a set of sharp or flat symbols placed together on the staff. Key signatures are generally written immediately after the clef at the beginning of a line of musical notation, although they can appear in other parts of a score, notably after a double barline. A key signature designates notes that are to be played higher or lower than the corresponding natural notes and applies through to the end of the piece or up to the next key signature. A sharp symbol on a line or space in the key signature raises the notes on that line or space one semitone above the natural, and a flat lowers such notes one semitone. Further, a symbol in the key signature affects all the notes of one letter: for instance, a sharp on the top line of the treble staff applies to Fs not only on that line, but also to Fs in the bottom space of the staff, and to any other Fs. This convention was not universal until the late Baroque/early Classical period, however; music published in the 1720s and 30s, for example, uses key signatures showing sharps or flats on both octaves for notes which fall within the staff. An accidental is an exception to the key signature, applying only in the measure in which it appears. Although a key signature may be written using any combination of sharp and flat symbols, about a dozen diatonic key signatures are by far the most common, and their use is assumed in much of this article. A piece scored using a single diatonic key signature and no accidentals contains notes of at most seven of the twelve pitch classes, which seven being determined by the particular key signature. Each major and minor key has an associated key signature that sharpens or flattens the notes which are used in its scale. However, it is not uncommon for a piece to be written with a key signature that does not match its key, for example, in some Baroque pieces,〔Schulenberg, David. ''Music of the Baroque''. New York: Oxford University Press, 2001. (p. 72. ). "(…) to determine the key of a Baroque work one must always analyze its tonal structure rather than rely on the key signature."〕 or in transcriptions of traditional modal folk tunes.〔Cooper, David. ''The Petrie Collection of the Ancient Music of Ireland''. Cork: Cork University Press, 2005. (p. 22 ). "In a few cases Petrie has given what is clearly a modal melody a key signature which suggests that it is actally in a minor key. For example, ''Banish Misfortune'' is presented in D minor, although it is clearly in the Dorian mode."〕 Later on, this use of a key signature that is theoretically incorrect for a piece as a whole or a self-contained section of a piece became less common (in contrast to brief passages within a piece, which, as they modulate from key to key often temporarily disagree with the key signature); but it can be found at least as late as one of Beethoven's very late piano sonatas. For example, in his Sonata no. 31 in A major, Op. 110, the first appearance of the Arioso section in the final movement is notated throughout in 6 flats; but it both begins and ends in A minor and has a significant modulation to C major, and both these keys theoretically require 7 flats in their key signature. (The second appearance later in the movement of this same section, a semitone lower, in G minor, uses the correct key signature of two flats.) == Conventions == In principle, any piece can be written with any key signature, using accidentals to correct the pattern of whole and half steps. The purpose of the key signature is to minimize the number of such accidentals required to notate the music. The sequence of sharps or flats in key signatures is generally rigid in modern music notation. For example, if a key signature has only one sharp, it must be an F sharp.〔|url=http://www.dummies.com/how-to/content/how-to-read-key-signatures.html |title=How to Read Key Signatures|accessdate=29 January 2014〕 However, in 20th-century music, there are occasional exceptions to this, where a piece uses an unorthodox or synthetic scale, where a key signature may be invented to reflect this. This may consist of a number of sharps or flats that are not the normal ones (such as a signature of just C or E), or it may consist of one or more sharps combined with one or more flats (such as a signature containing both F and B. Key signatures of this kind can be found in the music of Béla Bartók, for example. The effect of a key signature continues throughout a piece or movement, unless explicitly cancelled by another key signature. For example, if a five-sharp key signature is placed at the beginning of a piece, every A in the piece in any octave will be played as A sharp, unless preceded by an accidental (for instance, the A in scale (2) illustrated right — the next-to-last note — is played as an A even though the A in the key signature (the last sharp sign) is written an octave lower).〔 In a score containing more than one instrument, all the instruments are usually written with the same key signature. Exceptions include: * If an instrument is a transposing instrument * If an instrument is a percussion instrument with indeterminate pitch * Composers usually omit the key signature for timpani parts. Besides not using a key signature, timpani parts were early on also treated often as transposing instrument parts, the pitch of the high drum being written as C and, as timpani were almost always tuned a 4th apart, dominant on the low drum and tonic on the high drum, the pitch of the low drum being written as G, with the actual pitch indicated at the beginning of the part, e.g. timpani in D-A, if they were tuned A (low drum) and D (high drum) * Composers may omit the key signature for horn and occasionally trumpet parts. This is perhaps reminiscent of the early days of brass instruments, when crooks would be added to them, in order to change the length of the tubing and allow playing in different keys. * On occasion, in more modern works, instruments may be notated in different keys even when they are not transposing instruments, because the music is polytonal and the different parts are actually in different keys which sound together. 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 signature」の詳細全文を読む スポンサード リンク
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