Key signature
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.
For use in cryptography, see Key signature (cryptography).
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 end 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,[1] or transcriptions of traditional modal folk tunes.[2]
Major scale structure[edit]
Scales with sharp key signatures[edit]
There can be up to seven sharps in a key signature, appearing in this order: F♯ C♯ G♯ D♯ A♯ E♯ B♯.[4][5] The key note or tonic of a piece in a major key is a semitone above the last sharp in the signature.[6] For example, the key of D major has a key signature of F♯ and C♯, and the tonic (D) is a semitone above C♯. Each scale starting on the fifth scale degree of the previous scale has one new sharp, added in the order shown.[5]
History[edit]
The use of a one-flat signature developed in the Medieval period, but signatures with more than one flat did not appear until the 16th century, and signatures with sharps not until the mid-17th century.[8]
When signatures with multiple flats first came in, the order of the flats was not standardized, and often a flat appeared in two different octaves, as shown at right. In the late 15th and early 16th centuries, it was common for different voice parts in the same composition to have different signatures, a situation called a partial signature or conflicting signature. This was actually more common than complete signatures in the 15th century.[9] The 16th-century motet Absolon fili mi by Pierre de La Rue (formerly attributed to Josquin des Prez) features two voice parts with two flats, one part with three flats, and one part with four flats.
Baroque music written in minor keys often was written with a key signature with fewer flats than we now associate with their keys; for example, movements in C minor often had only two flats (because the A♭ would frequently have to be sharpened to A♮ in the ascending melodic minor scale, as would the B♭).