In Western musical notation, a key signature is a set of sharp (), flat (), or rarely, natural () symbols placed on the staff at the beginning of a section of music. The initial key signature in a piece is placed immediately after the clef at the beginning of the first line. If the piece contains a section in a different key, the new key signature is placed at the beginning of that section.
In a key signature, a sharp or flat symbol on a line or space of the staff indicates that the note represented by that line or space is to be played a semitone higher (sharp) or lower (flat) than it would otherwise be played. This applies through the rest of the piece or until another key signature appears. Each symbol applies to comparable notes in all octaves—for example, a flat on the fourth space of the treble staff (as in the diagram) indicates that all notes notated as Es are played as E-flats, including those on the bottom line of the staff.
Most of this article addresses key signatures that represent the diatonic keys of Western music. These contain either flats or sharps, but not both, and the different key signatures add flats or sharps according to the order shown in the circle of fifths.
Each major and minor key has an associated key signature, showing up to seven flats or seven sharps, that indicates the notes used in its scale. Music was sometimes notated with a key signature that did not match its key in this way—this can be seen 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 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 actually in a minor key. For example, Banish Misfortune is presented in D minor, although it is clearly in the Dorian mode."
Overview With any note as a starting point, a certain series of intervals produces a major scale: whole step, whole, half, whole, whole, whole, half.
Starting on C, this yields C–D–E–F–G–A–B–C (a C-major scale). There are no sharps or flats in this scale, so the key signature for C has no sharps or flats in it. Starting on any other note requires that at least one of these notes be changed (raised or lowered) to preserve the major scale pattern. These raised or lowered notes form the key signature. Starting the pattern on D, for example, yields D–E–F–G–A–B–C–D, so the key signature for D major has two sharps—F and C.
Key signatures indicate that this applies to the section of music that follows, showing the reader which key the music is in, and making it unnecessary to apply accidentals to individual notes.
In standard music notation, the order in which sharps or flats appear in key signatures is uniform, following the circle of fifths: F, C, G, D, A, E, B, and B, E, A, D, G, C, F. Musicians can identify the key by the number of sharps or flats shown, since they always appear in the same order. A key signature with one sharp must show F-sharp, which indicates G major or E minor.
There can be exceptions to this, especially in 20th-century music, if a piece uses an unorthodox or synthetic scale and an invented key signature to reflect that. This may consist of sharps or flats that are not in the usual order, or of sharps combined with flats (e.g., F and B). Key signatures of this kind can be found in the music of Béla Bartók, for example.
In a score, transposing instruments will show a different key signature to reflect their transposition but their music is in the same concert key as the other instruments. Percussion instruments with indeterminate pitch will not show a key signature, and timpani parts are sometimes written without a key signature (early timpani parts were sometimes notated with the high drum as "C" and the low drum a fourth lower as "G", with actual pitches indicated at the beginning of the music, e.g., "timpani in D–A"). In polytonal music, where different parts are actually in different keys sounding together, instruments may be notated in different keys.
Conventions The order in which sharps or flats appear in key signatures is illustrated in the diagram of the circle of fifths. Starting the major scale pattern (whole step, whole, half, whole, whole, whole, half) on C requires no sharps or flats. Proceeding clockwise in the diagram starts the scale a fifth higher, on G. Starting on G requires one sharp, F, to form a major scale. Starting another fifth higher, on D, requires F and C. This pattern continues, raising the seventh scale degree of each successive key. As the scales become notated in flats, this is shown by eliminating one of the flats. This is strictly a function of notation—the seventh scale degree is still being raised by a semitone compared to the previous key in the sequence. Going counter-clockwise from C results in lowering the fourth scale degree with each successive key (starting on F requires a B to form a major scale). Each major key has a relative minor key that shares the same key signature. The relative minor is always a minor third lower than its relative major.
The key signatures with seven flats and seven sharps are usually notated in their enharmonic equivalents. C major (seven sharps) is usually written as D major (five flats) and C major is usually written as B major.
The key signature may be changed at any time in a piece by providing a new signature. If the new signature has no sharps or flats, a signature of naturals, as shown, is used to cancel the preceding signature. If a change in signature occurs at the start of a new line on the page, where a signature would normally appear, the new signature is customarily repeated at the end of the previous line to make the change more conspicuous.
Variations
When a key signature changes from sharps to flats or vice versa, the old key signature can be cancelled with a matching set of naturals before the new key signature is shown. This was a frequent convention in older notation styles but newer music and newer editions of old music sometimes simply show the new signature without the canceling naturals. These naturals are necessary, however, if the new key signature has no sharps or flats (C major, A minor, or an open key signature).
Similarly, when a flat key changes to fewer flats, or a sharp key changes to fewer sharps, the older convention was to use naturals to cancel the flats or sharps being eliminated before the new signature is written. Modern usage often simply shows the new signature without these naturals. The use of natural signs is still required if the new key signature has no sharps or flats.
When a flat key changes to more flats or a sharp key changes to more sharps, the new signature is simply written in without using naturals to cancel the old signature. This convention applies in both traditional and newer styles.
At one time it was usual to precede the new signature with a double barline even if it was not otherwise required, but it has become increasingly common to simply use a single barline. The courtesy signature that appears at the end of a line immediately before a change is usually preceded by an additional barline and the line at the very end of the staff is omitted.
If both naturals and a new key signature appear at a key signature change, there are also modern variations about where a barline will be placed. In some scores by Debussy the barline is placed after the naturals but before the new key signature. Hitherto, it would have been usual to place all the symbols after the barline.
The A which is the fifth sharp in the sharp signatures may occasionally be notated on the top line of the bass staff, whereas it is more usually found in the lowest space on that staff. An example of this can be seen in the full score of Ottorino Respighi's Pines of Rome, in the third section, "Pines of the Janiculum" (which is in B major), in the bass-clef instrumental parts.
In the case of seven-flat key signatures, the final F may occasionally be seen on the second-top line of the bass staff, whereas it would more usually appear on the space below the staff. An example of this can be seen in Isaac Albéniz's Iberia: first movement, "Evocación", which is in A minor.
Double flats and sharps Key signatures can extend into double flats or double sharps, but this is extremely rare. For example, the key of G major would have eight sharps, requiring an F double-sharp (F) and six single sharps. The key of A major, with four flats, is enharmonically equivalent and would generally be used instead. Although the notation of key signatures could theoretically extend beyond single flats or sharps, a key signature with more than one or two double flats or sharps would be exceedingly hard to read and would be replaced by the enharmonically equivalent key signature. {| class="wikitable" !Major !Key signature !Minor |- |F major (E major) |8 flats (4 sharps) |D minor (C minor) |- |B major (A major) |9 flats (3 sharps) |G minor (F minor) |- |E major (D major) |10 flats (2 sharps) |C minor (B minor) |- |A major (G major) |11 flats (1 sharp) |F minor (E minor) |- |D major (C major) |12 flats (no sharps or flats) |B minor (A minor) |- |G major (A major) |8 sharps (4 flats) |E minor (F minor) |- |D major (E major) |9 sharps (3 flats) |B-sharp minor|B minor (C minor) |- |A major (B major) |10 sharps (2 flats) |F minor (G minor) |- |E major (F major) |11 sharps (1 flat) |C minor (D minor) |- |B major (C major) |12 sharps (no flats or sharps) |G minor (A minor) |} A piece in a major key might modulate up a fifth to the dominant (a common occurrence in Western music), resulting in a new key signature with an additional sharp. If the original key was C-sharp, such a modulation would lead to the key of G-sharp major (with eight sharps) requiring an F in place of the F. This section could be written using the enharmonically equivalent key signature of A-flat major instead. Claude Debussy's Suite bergamasque does this: in the third movement "Clair de lune" the key shifts from D-flat major to D-flat minor (eight flats) for a few measures but the passage is notated in C-sharp minor (four sharps); the same happens in the final movement, "Passepied", in which a G-sharp major section is written as A-flat major.
Such passages may instead be notated with the use of double-sharp or double-flat accidentals. This passage in G-sharp major is from the C-sharp-major fugue of Book 1 of Johann Sebastian Bach's Well-Tempered Clavier.
While these keys are sometimes notated with accidentals as in this example, the use of a key signature containing double flats or sharps is very rare. The final pages of John Foulds' A World Requiem are written in G♯ major (with F in the key signature), No. 18 of Anton Reicha's Practische Beispiele is written in B major, and the third movement of Victor Ewald's Brass Quintet Op. 8 is written in F♭ major (with B in the key signature).
Examples of such key signatures are pictured below:
There does not appear to be a standard on how to notate these key signatures:
The default behaviour of LilyPond (pictured above) writes all single sharps or flats in the circle-of-fifths order before showing the double signs. This is the format used in John Foulds' A World Requiem, Op. 60, which uses the key signature of G♯ major as displayed above. The sharps in the key signature of G♯ major proceed C♯, G♯, D♯, A♯, E♯, B♯, F.