A key signature serves as a guide for the performer of a piece of music. The key signature determines the pattern of accidentals -- sharps and flats -- to be played, and reappears at the beginning of each staff. (In this way it differs from a time signature, which need be written only once). For example, if the key signature consists of only F-sharp, each written note F in the piece should be played as F-sharp, even though no sharp immediately precedes the written note. Often, however, an accidental contradicting the direction of the key signature will appear immediately before the written note. An accidental immediately preceding the written note always takes precedence. (For instance, in our example, if the key signature contains F-sharp but a written note F appears with a natural sign written in front of it, F-natural should be played). In traditional notation, this influence of the new accidental applies for the duration of the measure before the key signature resumes its "dominion", or until canceled by a subsequent new accidental.

Since a the presence of a "key" in music is contingent on a certain pattern of accidentals, the key signature gives a partial indication of the key of a given passage of music. We need more than the key signature to determine the key of a passage, though -- that must be decided on the basis of contextual features to be explained elsewhere in the School of Music. All other things being equal, though, each key signature can represent one of two musical keys: one major, and one minor. Each is termed the "relative" major or minor, respectively, in relation to the other. For example, B-flat major and G minor have the same key signature: the relative minor of the key of B-flat major is G minor, while B-flat major is the relative major of G minor. The following tables illustrate each key signature, along with a listing of their corresponding keys. It is of paramount importance for the practicing musician to memorize these keys and to be able to recall them instantly.

Key Sig.Major KeyMinor Key
C Major key signature
No sharps or flats
C major A minor
Key Sig.Major KeyMinor Key
F Major key signature
1 flat
F major D minor
B-flat Major key signature
2 flats
B♭ major G minor
E-flat Major key signature
3 flats
E♭ major C minor
A-flat Major key signature
4 flats
A♭ major F minor
D-flat Major key signature
5 flats
D♭ major B♭ minor
G-flat Major key signature
6 flats
G♭ major E♭ minor
C-flat Major key signature
7 flats
C♭ major A♭ minor
Key Sig.Major KeyMinor Key
G Major key signature
1 sharp
G major E minor
D Major key signature
2 sharps
D major B minor
A Major key signature
3 sharps
A major F# minor
E Major key signature
4 sharps
E major C# minor
B Major key signature
5 sharps
B major G# minor
F-sharp Major key signature
6 sharps
F# major D# minor
C-sharp Major key signature
7 sharps
C# major A# minor