How to work out the key of a piece of music using sharps
In music theory exams you may be asked to work out a key from a piece of music with accidentals. In this example, I am going to use sharps.
Question: What is the key of the following piece of music?
When the key is a major key
Make a list of all the sharps which appear in the example above (Some are duplicated so there is no need to write those):
C sharp; F sharp.
Put the sharps in the order they would appear in a key signature. I use the phrase Fat Cats Go Down And Eat Bacon
F sharp (Fat); C sharp (Cats)
We have now discovered there are 2 sharps. Look at the last sharp, C sharp and move up one semitone to D. This means that D major is the major key of the example above.
When the key is a minor key
Write down the order of sharps as they appear in the example above. There is no need to write down any sharps which are duplicated
A sharp; F sharp; C sharp;
Write down the sharps in the order they would appear in a key signature. I use the phrase Fat Cats Go Down And Eat Bacon.
F sharp; C sharp; A sharp.
In the list above, there are some sharps missing between C sharp and A sharp. The list is not complete because sharps ALWAYS appear in the same order in a key signature. Where is G sharp and D sharp? In the example, G and D do not have sharps but appear instead as G and D natural. The sharps are out of sequence so this should alert you to the idea that the example may not be in a major key. F and C sharp are both in the correct sequence but A sharp may be an accidental.
Looking at the last sharp which appears in the correct sequence, C sharp, move up one semitone to D. This means that if the example was in a major key, it would be D major
Work out the relative minor of D major, which has 2 sharps, F sharp and C sharp. The 6th note of the D major scale is B so B minor is the relative minor. Working out the notes of B harmonic minor we have: B C# D E F# G A# B
The 7th, leading note, of B harmonic minor is A sharp.
We can now see that in the example above, the key must be B minor and not D major
When working out whether a key is major or minor, the biggest clue is the 7th or leading note of the minor key, which is always raised by one semitone in the harmonic minor form.