Work out the key of a piece of music – sharps

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

Step 1:

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.

Step 2:

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)

Step 3:

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

Step 1:

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;

Step 2:

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.

Step 3:

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

Step 4:

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.

Step 4:

We can now see that in the example above, the key must be B minor and not D major

Note:

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.  

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