A key signature is the group of sharps or flats at the start of a staff that tells you which notes are sharp or flat throughout a piece. C major has no sharps or flats, G major has one sharp, F#, and F major has one flat, Bb, and the key signature tells you the key at a glance.
What is a key signature?
A key signature is the group of sharps or flats written at the start of a staff, just after the clef, that tells you which notes are sharp or flat throughout a piece of music. Instead of writing a sharp sign before every F# in a piece in G major, the composer writes a single sharp on the F line at the start of the staff, and every F in the piece is played as F# unless a natural sign cancels it. The key signature is a shorthand that saves ink and makes the music easier to read, and it also tells the player the key at a glance. Every major key has a different key signature, and every minor key shares the key signature of its relative major. C major and A minor have no sharps and no flats. G major and E minor have one sharp, F#. D major and B minor have two sharps, F# and C#. F major and D minor have one flat, Bb. The number of sharps or flats tells you how far around the circle of fifths the key sits from C major, and the specific sharps or flats tell you which notes are altered. The key of G major and the key of F major show two key signatures, and the circle of fifths shows them all.
How key signatures work
The sharps in a key signature always appear in the same order, F#, C#, G#, D#, A#, E#, B#, which is the order of the circle of fifths, and the flats always appear in the reverse order, Bb, Eb, Ab, Db, Gb, Cb, Fb. That fixed order is what lets you read a key signature quickly: if you see two sharps, they must be F# and C#, so the key is D major or B minor; if you see three flats, they must be Bb, Eb and Ab, so the key is Eb major or C minor. There is no key signature with a random collection of sharps and flats, because the order is fixed by the circle. To work out the key from a key signature, use two shortcuts. For sharp keys, the last sharp in the signature is the 7th degree of the key, so the key is a semitone above it: if the last sharp is F#, the key is G major; if the last sharp is C#, the key is D major. For flat keys, the second-to-last flat is the key itself: if the flats are Bb and Eb, the key is Bb major, because Bb is the second-to-last flat; if the flats are Bb, Eb and Ab, the key is Eb major. The one exception is F major, which has a single flat, Bb, and no second-to-last flat to read. The scale finder works out the key from any signature, and the key finder finds the key of a set of notes.
Worked example — reading a key signature of four sharps. Four sharps must be F#, C#, G# and D#, in that order, because the order of sharps is fixed. The last sharp is D#, and the key is a semitone above it, E, so the key is E major, or its relative minor, C# minor. Check it against the E major scale, E, F#, G#, A, B, C#, D#, which contains exactly those four sharps. Now read a key signature of three flats: the flats must be Bb, Eb and Ab, and the second-to-last flat is Eb, so the key is Eb major, or its relative minor, C minor. The E major scale and the C minor scale show the two keys.
How to practise reading key signatures
Practise reading key signatures by covering the key name and working it out from the sharps or flats, so the shortcuts become automatic.
- Write out the key signatures of G, D, A and E major, in the order of sharps, and check that each one adds one sharp to the previous key.
- Look at a key signature with three sharps and work out the key using the last-sharp shortcut, before you check your answer.
- Look at a key signature with four flats and work out the key using the second-to-last-flat shortcut, before you check your answer.
- Play the scales of the key of D major and the key of Bb major and notice how the key signature shapes the notes you play.
- Use the circle of fifths tool to check the key signature of any key, and repeat the exercise until you can read any signature in seconds.
Common mistakes
- Reading the sharps in the wrong order — The sharps in a key signature always appear in the order F#, C#, G#, D#, A#, E#, B#, and the flats always appear in the order Bb, Eb, Ab, Db, Gb, Cb, Fb. If you read them in a different order, you will name the wrong key.
- Forgetting that the key signature applies to every octave — A sharp in the key signature applies to every F in the piece, in every octave, not just the F on the line where the sharp is written. The same is true for flats, so check every note against the signature.
- Confusing the major key with its relative minor — A key signature with one sharp could be G major or E minor, because they share the signature. Look at the music itself, the final chord and the accidentals, to tell which one it is.
Quick reference
This table lists the key signatures from seven sharps to seven flats, with the major key and its relative minor for each signature, so you can see the whole system in one place. The sharp signatures run down the left side of the circle of fifths and the flat signatures run down the right side, and they meet at the bottom, where six sharps and six flats describe the same sounding key.
| Key signature | Major key | Relative minor |
|---|---|---|
| 7 sharps | C# major | A# minor |
| 6 sharps | F# major | D# minor |
| 5 sharps | B major | G# minor |
| 4 sharps | E major | C# minor |
| 3 sharps | A major | F# minor |
| 2 sharps | D major | B minor |
| 1 sharp | G major | E minor |
| No sharps or flats | C major | A minor |
| 1 flat | F major | D minor |
| 2 flats | Bb major | G minor |
| 3 flats | Eb major | C minor |
| 4 flats | Ab major | F minor |
Memorise the table by reciting the sharp keys in order, C, G, D, A, E, B, F#, C#, and then the flat keys in order, F, Bb, Eb, Ab, Db, Gb, Cb. When you can recite both lists without looking, you can read any key signature in seconds, and you will never need to count sharps and flats again.
Frequently asked questions
What is a key signature?
A key signature is the group of sharps or flats at the start of a staff that tells you which notes are sharp or flat throughout a piece. C major has no sharps or flats, G major has one sharp, F#, and F major has one flat, Bb.
How do I read a key signature?
For sharp keys, the last sharp is the 7th degree of the key, so the key is a semitone above it. For flat keys, the second-to-last flat is the key itself. The sharps always appear in the order F#, C#, G#, D#, A#, E#, B#, and the flats in the order Bb, Eb, Ab, Db, Gb, Cb, Fb.
Why do key signatures exist?
To save writing: instead of marking every sharp or flat note in a piece, the composer writes the sharps or flats once, at the start of the staff, and they apply to every note of that name in the piece. The key signature also tells the player the key at a glance.
Do minor keys have their own key signatures?
No. A minor key shares the key signature of its relative major, so A minor has no sharps or flats, like C major, and E minor has one sharp, like G major. The key signature alone cannot tell you whether a piece is major or minor.
What is the order of sharps?
F#, C#, G#, D#, A#, E#, B#, which is the order of the circle of fifths. The order of flats is the reverse: Bb, Eb, Ab, Db, Gb, Cb, Fb, which is the order of the circle of fourths.