beginning of a line of musical notation and can appear also in other parts of the score, for
example, after a double bar.
The purpose of the key signature is to minimise the number of accidentals required to
notate the music. In theory any piece can be written with any key signature, using
accidentals to correct any notes where it should not apply. The absence of a key
signature does not always mean that the piece is in the key of C major or A minor. Each
accidental may be notated explicitly, or the piece may be modal, or it may be atonal.
The effect of a key signature continues throughout a piece, unless cancelled by another
key signature. If a five sharp key signature is placed at the beginning of a piece every A
in the piece in any octave must be played as A sharp unless it is preceded by an
accidental in the form a natural sign. If there is only one sharp it must be F sharp. The
sequence of sharps or flats in key signatures is rigid.
A key signature is not the same as a key. Key signatures are merely notational devices.
They are convenient for diatonic or tonal music. Some pieces that modulate, that is,
change key, insert a new key signature on the staff while others use natural signs to
neutralise the key signature and other sharps or flats for the new key.
The key signature defines the diatonic scale that a piece of music uses. Most scales
require that some notes be consistently sharpened or flattened. In the key of G major the
leading note is F sharp so the key signature for G major is the one sharp signature. There
is no causal connection, however, and a piece with a one sharp key signature is not
necessarily in the key of G major. Many other factors determine the key of a piece. This
is particularly true of minor keys. The ‘Dorian’ Toccata and Fugue by Bach is in D
minor but here is no key signature thus implying that it is in the key of C major. Instead
the B flats necessary for the key of D minor are written as accidentals wherever necessary.
Two keys which share the same key signature are called relative keys. When musical
modes, such as Lydian or Dorian, are written using key signatures they are called
transposed modes.
Major keys with sharps
C major has no sharps or flats
G major has one sharp: F
D major has two sharps: F C
A major has three sharps: F C G
E major has four sharps: F C G D
B major has five sharps: F C G D A
F sharp has six sharps: F C G D A E
C sharp has seven sharps F C G D A E B
FCGDAEB may be remembered by the mnemonic Fat Charlie Got Down After Eating
Bananas.