When you need to work out what key some music is in, there are some important rules to follow and some common traps avoid!
Use the key signature – this narrows the choice down to one major key and one minor key
Find clues in the harmonic structure
Find clues in the notes which are emphasized
Don’t rely on accidentals
Use the Key Signature to Work out the KeyÂ
Each key signature represents one major and one minor key.
| 0 flats/sharps |
| C major |
| A minor |
| 1 sharp | 2 sharps | 3 sharps | 4 sharps | 5 sharps | 6 sharps | 7 sharps |
| G major | D major | A major | E major | B major | F# major | C# major |
| E minor | B minor | F# minor | C# minor | G# minor | D# minor | A# minor |
| 1 flat | 2 flats | 3 flats | 4 flats | 5 flats | 6 flats | 7 flats |
| F major | Bb major | Eb major | Ab major | Db major | Gb major | Cb major |
| D minor | G minor | C minor | F minor | Bb minor | Eb minor | Ab minor |
Use the Harmonic Structure to work out whether it’s Major or Minor
Key is fixed by the use of the tonic and dominant chords. The tonic is chord I, and the dominant is chord V.
In C major, chord I is C-E-G. Chord V is made from the 5th note in the C major scale, which is G. Chord V is G-B-D.
In A minor, chord I is A-C-E and chord V is E-G#-B. (We raise the leading note to G#, as in the harmonic minor scale).
The first full bar of the melody will usually use notes which belong to chords I and/or V. (You can also look at any upbeats, and at bar 2 as well, but bar 1 is the most important).
These notes will normally fall on the beat (not in between beats).
Here is an example.
What key is this in? There is one sharp. Is it G major or E minor?
Chords I and V in G major are: G-B-D & D-F#-A
Chords I and V in E minor are: E-G-B & B-D#-F#
The time signature is 2/4, so the main beats are the notes which fall on the 1st and 2nd beats of the bar. These notes are boxed in red.
The notes in the boxes are G, D and G.
The D natural shows us that it is G major. D natural does not occur in chord I or V in E minor.
Here is another example (in case you thought the lack of D# was enough to prove it is G major).
What key is this? There is one flat. Is it F major or D minor?
Chords I and V in F major are F-A-C & C-E-G
Chords I and V in D minor are: D-F-A & A-C#-E
The time signature is 3/4, so we look at the notes on the 1st, 2nd and 3rd beats of the bar.
The notes on the beat are D, F, D and F.
D doesn’t appear in chord I or V in F major. The key is D minor even though there are no accidentals!
Use the Emphasized Notes to Help You
There are many ways to emphasize a note. The tonic and dominant notes are often emphasized in these ways:
Played on the first beat of the bar
Played loudly
Played with an accent
Repeated
Look at this example. Is the key E major or C# minor?
It is C# minor.
Don’t Rely on Accidentals
This is a very common error. Many people think that because there is an accidental in the harmonic minor scale, that there must also be accidentals in the minor key. This is not true:
Melodies are built using the melodic minor scale. This has both sharpened and unsharpened notes. For example, A minor melodic contains the notes F natural, F sharp, G natural and G sharp.
Only the 6th and 7th notes of the scale are affected. Some melodies don’t use these notes.
Accidentals can be added to a piece for decoration, without affecting the key.
Here is an example.
What key is this in? There are no sharps or flats in the key signature. Is it C major or A minor?
 The notes which fall on the beat (starting from the upbeat) are G-C-E. They belong to chord I in C major.
 The G# is just to decorate the melody. It does not affect the key of the piece. Listen to the extract for yourself!