A key signature is a set of sharps or flats written at the beginning of a staff that tells you which notes are altered throughout the piece. Instead of marking every F as F# individually, you write one sharp at the beginning and every F in the piece is played sharp.
How Key Signatures Work
Key signatures appear right after the clef on the staff. The sharps or flats are placed in a specific order on specific lines and spaces, and this order is always the same regardless of the key:
- Order of sharps: F# – C# – G# – D# – A# – E# – B#
- Order of flats: B♭ – E♭ – A♭ – D♭ – G♭ – C♭ – F♭
The number of sharps or flats tells you which key you're in.
Sharp Keys
| Sharps | Major Key | Relative Minor |
|---|---|---|
| 0 | C major | A minor |
| 1 (F#) | G major | E minor |
| 2 (F#, C#) | D major | B minor |
| 3 | A major | F# minor |
| 4 | E major | C# minor |
| 5 | B major | G# minor |
| 6 | F# major | D# minor |
| 7 | C# major | A# minor |
Flat Keys
| Flats | Major Key | Relative Minor |
|---|---|---|
| 1 (B♭) | F major | D minor |
| 2 (B♭, E♭) | B♭ major | G minor |
| 3 | E♭ major | C minor |
| 4 | A♭ major | F minor |
| 5 | D♭ major | B♭ minor |
| 6 | G♭ major | E♭ minor |
| 7 | C♭ major | A♭ minor |
Quick Tricks for Reading Key Signatures
For sharps: The last sharp in the key signature is the leading tone (the 7th degree). The key is one half step above it. If the last sharp is F#, the key is G major.
For flats: The second-to-last flat in the key signature is the tonic. If you see B♭ and E♭, the key is B♭ major. (The exception: one flat = F major.)
Major vs Minor Keys
Every key signature represents two keys — a major key and its relative minor. Both use the same notes. G major and E minor share one sharp (F#). To tell which one a piece is actually in, look at where the music gravitates. Does it feel resolved on G or on E?
Why Key Signatures Matter
Key signatures exist to reduce clutter on the page. More practically, understanding them tells you immediately which notes are "in" the key — so you know what chords are available, which notes fit a melody, and how far you are from other related keys. The circle of fifths visualizes all key signatures and their relationships at a glance.