Key Signatures and Key Names

The vast majority of music is written in a specific key that can be figured out by looking at the key signature.  The initial key of a song is determined by looking at the first measure of the sheet music between the clef sign and the time signature.  The image below points out the location of the key signature.

Naming The Key When There Are Sharps In The Key Signature
If you have 1 or more sharps in the key signature, the rule for determining the name of the key is to find the last sharp and then go up a half step.  The sharps will always be in order from left to right, so the last sharp will always be the one farthest to the right.  In the example below, the last sharp (B#) is highlighted in red:
If you play a B# on the piano and then play the note a half step higher, you will land on a C#.  Therefore, this key signature represents the key of C# Major or its relative minor key of A# minor.  To determine if your song (or section of music) is written in the major or relative minor key, look at the last note(s) or chord(s) in the last measure.  Most of the time, the notes in the last measure will either belong to the major or minor representation of the key signature.  Below is a table of last sharp names and the major/minor keys that the key signature defines:
Last Sharp(#) Major Key Minor Key
F# G Major E Minor
C# D Major B Minor
G# A Major F# Minor
D# E Major C# Minor
A# B Major G# Minor
E# F# Major D# Minor
B# C# Major A# Minor

Naming The Key When There Are Flats In The Key Signature
The general rule for determining the key when there are flats in the key signature is to find the 2nd to last flat, and the name of that flat will be the name of the major key.  The flats will be in order from left to right.  In the example below, the 2nd to last flat is highlighted in blue:

The flat highlighted is E♭, so this key signature represents E♭ Major or its relative minor key of C minor.

Exceptions To The Rules
  • No Sharps/No Flats.  If there aren’t any sharp or flat signs in the key signature, then you can’t apply either rule.  This key signature is either C Major or A minor.
  • One Flat.  The general rule for flats doesn’t work if there’s only 1 flat in the key signature.  The key will be either F Major or D minor.
You may also find the following links helpful:

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