Key signature

For use in cryptography see Key signature (cryptography)

In musical notation, a key signature is a series of sharp symbols or flat symbols placed on the staff, designating notes that are to be played one semitone higher or lower unless otherwise noted with an accidental. 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, usually after a double bar.

Here is a B major scale written with accidentals:

Missing image
Ks1.gif
Image:ks1.gif

and here is the same scale (played on the same notes) written using a key signature:

Missing image
Ks2.gif
Image:ks2.gif

The effect of a key signature continues throughout a piece or movement, unless explicitly cancelled by another key signature. For example, if a one-sharp key signature is placed at the beginning of a piece, every F in the piece in any octave will be played as F sharp, unless there is an accidental. (If there is only one sharp, it must be F sharp; the sequence of sharps or flats in key signatures is rigid. See below.)

In a score containing more than one instrument, all the instruments are usually written with the same key signature. Exceptions:

• If an instrument is a transposing instrument
• If an instrument is a percussion instrument with indeterminate pitch
• In 15th-century scores partial signatures are quite common, in which different voices will have different key signatures.

Relation of signature to key

A key signature is not the same as a key; key signatures are merely notational devices. They are convenient principally for diatonic or tonal music. Some pieces which change key (modulate) insert a new key signature on the staff partway, while others use accidentals: natural signs to "neutralize" the key signature and other sharps or flats for the new key.

For a given mode the key signature defines the diatonic scale which a piece of music uses. Most scales require that some notes be consistently sharpened or flattened. For example, in the key of G major, the leading-note is F sharp. So the key signature associated with G major is the one-sharp key signature. However, there is no causal connection; if you see a piece with a one-sharp key signature, you cannot be certain it is in G major. Many other factors determine the key of a piece. This is particularly true of minor keys. The famous Dorian toccata by Bach is so named because, although it is in D minor, there is no key signatures, as if it were in D dorian. Instead, the B flats necessary for D minor are written as accidentals.

Two keys which share the same key signature are called relative keys.

When modes such as Lydian or Dorian are written using key signatures, they are called transposed modes.

History

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. 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.

Table of key signatures

The table below illustrates the number of sharps or flats for each key signature and the relative major key signatures for minor scales (see circle of fifths). Remembering all the key signatures is easily done when you apply four simple rules:

• No sharps or flats is C major
• One flat if F major
• For more than one flat, the (major) key is the next-to-last flat.
• For any number of sharps, take thelast sharp and go up one semi-tone to get the (major) key.

(The relative minor is a minor 3rd down from the major).

For key signatures with sharps, the first sharp is placed on F line (for the key of G major/E minor). Subsequent additional sharps are added on C, G, D, A, E and B. For key signatures with flats, the first flat is placed on the B line, with subsequent flats on E, A, D, G, C and F. There are 15 different key signatures, including the "empty" signature of C major/A minor.

The key signatures with seven flats and seven sharps are very rarely used, because they have simpler enharmonic equivalents. For example, the key of C# major (seven sharps) is more simply represented as D♭ major (five flats). For modern practical purposes these keys are the same, because C# and D♭ are the same note. Pieces are written in these seven sharp or flat keys, however. The third Prelude and Fugue from Book One of Johann Sebastian Bach's Well-Tempered Clavier is in C# major, for example.

Nonstandard key signatures not included in the chart below are sometimes used by composers. One example is the key signature to Frederic Rzewski's song God to a Hungry Child (lyrics by Langston Hughes), which features B♭, E♭, and an F# in one key signature but which starts in the key of D on a D major chord.

Note that an absence of a key signature does not always mean that the music is in the key of C major or A minor: each accidental may be notated explicitly, or the piece may be atonal.

Key Sig.	Major Key	Minor Key
Missing image C_Major_key_signature.png C Major key signature No sharps or flats	C major	A minor

Template:Musical notationes:Armadura de clave fr:Armure (musique) ja:調号・臨時記号