Keys and the circle of fifths
Say exactly what "in the key of G" means, read a key signature at a glance, and use the circle of fifths to know how many sharps or flats any key has.
ONE WORD
What a key is
A key names the home of a song. "In G major" tells you three things at the same time: the home note is G, the notes come mainly from the G major scale, and the chords come from the family built on that scale, with G as I.
So in the key of G you expect G, C and D to do most of the work (I, IV and V), Em to turn up for the sad bit (vi), and every F in the piece to be played as F#.
Key signature
The sharps or flats written once at the start of every line, instead of in front of each note. They apply to that letter in every octave, all the way through the piece. G major has one sharp, F#. F major has one flat, Bb. C major has an empty key signature.
A song in G major might contain thirty F's. The key signature writes F# once, and all thirty are played on the black key.
Check yourself
Nika opens a piece of sheet music. One sharp sits at the start of the line, on the F. Halfway down the page she reaches a note on F with nothing written beside it. What does she play?
- F, because there is no sharp sign next to that note
- She must work out whether the piece is in G major or E minor before she can tell
- F first time and F# on the repeat
- F#, because the key signature covers every F in the piece
Show the answer
F#, because the key signature covers every F in the piece
Right. A key signature is a standing instruction, not decoration for the first bar. Every F on the page is F#, in any octave.
Two keys, two key signatures.
G major G A B C D E F# G signature: F#
F major F G A Bb C D E F signature: Bb
A bar of G major as it is written and as it is played:
key sig: F# | G A B C | D E F G |
you play: G A B C D E F# GOne sharp at the start of the line
changed one note in the second bar.This is the whole job of a key signature: write the sharp once instead of forty times. It is also why you can often name a song's key by glancing at the start of the line, before you read a single note.
Two sides of the same map
Sharp keys
Each one is a fifth up from the last, and each adds one sharp: C (none), G (F#), D (F# C#), A (three), E (four), B (five). The sharps always arrive in the order F C G D A E B.
Flat keys
Each one is a fifth down from the last, and each adds one flat: C (none), F (Bb), Bb (Bb Eb), Eb (three), Ab (four). The flats arrive in the reverse order: B E A D G C.
Check yourself
F major and G major are next-door neighbours on either side of C, so they use the same seven notes as each other.
Show the answer
False
They share only five notes. G major has F# and F major has Bb, and neither key contains the other's black key. The scales that really do share all seven notes are a major scale and its relative minor: G major and E minor, F major and D minor.
THE MAP
The circle of fifths
Line the keys up by fifths instead of alphabetically and something tidy happens. Start at C and go up a fifth each time: C, G, D, A, E, B. Every step adds exactly one sharp. Go the other way, down a fifth: C, F, Bb, Eb, Ab. Every step adds one flat. Drawn as a clock face with C at twelve o'clock, this is the circle of fifths.
Neighbours on the circle are the keys that sound closest to each other, because their scales differ by exactly one note. That is why a song can slip from C into G or F and barely feel like it moved.
Clockwise from C: up a fifth each time, one more sharp.
C (none)
G F#
D F# C#
A F# C# G#
E F# C# G# D#
B F# C# G# D# A#
Anti-clockwise from C: down a fifth each time, one more flat.
C (none)
F Bb
Bb Bb Eb
Eb Bb Eb AbSharps always appear in this order: F C G D A E B
Flats appear in the reverse order: B E A D G CC up to G is seven half steps, the perfect fifth from lesson 7. Nobody chose this order; it falls out of the scale pattern. Each new scale keeps six notes of the one before and changes one.
Check yourself
Omid is learning a piece in A major. How many sharps does its key signature have, and which ones?
- One: A#
- Two: F# and C#
- Three: F#, C# and G#
- Four: F#, C#, G# and D#
Show the answer
Three: F#, C# and G#
Yes. A is three steps clockwise from C (C to G to D to A), so three sharps, taken in order from F C G D A E B.
Check yourself
What is in each key's signature?
- G major
- F major
- C major
- D major
- Bb major
- A minor
Show the answer
Sharps: G major, D major
Flats: F major, Bb major
Nothing at all: C major, A minor
Work out what key a song is in
- Look at the start of the line
Count the sharps or flats and use the two-second trick: last sharp up a half step, or the second-from-last flat.
- Decide major or minor
One signature serves two keys. Look at where the song comes to rest. Last chord G and last melody note G means G major; resting on E means E minor.
- Check it against the chords
In G major you expect a lot of G, C, D and Em. A page full of Em and Am, with D leaning back towards Em, says E minor instead.
- No sheet music? Use your ear
Hum the note the song keeps coming home to and find it on a keyboard. That note, plus major or minor, is the key. Lesson 12 turns this into a proper method.
Check yourself
Match each key signature to the major key it belongs to
Show the answer
- Nothing at the start of the line → C major
- One sharp: F# → G major
- Two sharps: F# C# → D major
- One flat: Bb → F major
- Two flats: Bb Eb → Bb major
Lesson recap
- A key names a song's home note, the scale it draws on and the chord family built on that scale.
- The key signature writes the sharps or flats once at the start of the line; they apply to that letter everywhere.
- Clockwise round the circle of fifths adds a sharp each step: C, G, D, A, E, B. Anti-clockwise adds a flat: C, F, Bb, Eb, Ab.
- Sharps always come in the order F C G D A E B, flats in the reverse: B E A D G C.
- To read a signature: last sharp up a half step, or the second-from-last flat. One signature, two keys: G major or E minor.