A set of sharps or flats at the beginning of the staff that apply to the entire piece. Instead of marking every accidental individually, the key signature handles it once.

Sharps always appear in this order: F C G D A E B

Each new sharp key adds the next sharp in the sequence.

Flats always appear in this order: B E A D G C F

The reverse of the sharps.

Sharp Keys

Key	Sharps	Notes Sharped
C	0	—
G	1	F#
D	2	F# C#
A	3	F# C# G#
E	4	F# C# G# D#
B	5	F# C# G# D# A#
F#	6	F# C# G# D# A# E#
C#	7	F# C# G# D# A# E# B#

Flat Keys

Key	Flats	Notes Flatted
F	1	Bb
Bb	2	Bb Eb
Eb	3	Bb Eb Ab
Ab	4	Bb Eb Ab Db
Db	5	Bb Eb Ab Db Gb
Gb	6	Bb Eb Ab Db Gb Cb
Cb	7	Bb Eb Ab Db Gb Cb Fb

Sharp keys: the last sharp is one half step below the key. Last sharp is F#? You're in G. Last sharp is C#? You're in D.

Flat keys: the second-to-last flat is the key. Two flats (Bb, Eb)? You're in Bb. Three flats (Bb, Eb, Ab)? You're in Eb. Exception: one flat (Bb) = F major. Just memorize that one.

No sharps or flats? C major.

Every key signature represents two keys — a major key and its relative minor. They share the same notes.

Major	Relative Minor
C	Am
G	Em
D	Bm
A	F#m
E	C#m
B	G#m
F# / Gb	D#m / Ebm
F	Dm
Bb	Gm
Eb	Cm
Ab	Fm
Db	Bbm

The relative minor starts on the 6th degree of the major scale. Going the other way, the relative major starts on the 3rd degree of the natural minor.

At the bottom of the Circle of Fifths, three pairs of keys are the same pitches with different names:

Sharp Spelling	Flat Spelling
B (5#)	Cb (7b)
F# (6#)	Gb (6b)
C# (7#)	Db (5b)

In practice, the simpler spelling wins.