A Rule of Thumb for Key Signatures

Use the five fingers of the right hand only

Start with the thumb and tap each finger in turn until the little finger is tapped and then change direction and tap back to the thumb and so on, back and forth. At each change of direction you will actually say the name of the Key Signature and find it also indicates the number of sharps or flats it has.

Sharp Series

Each change of direction indicates the number of sharps in the key signature. The first change indicates one sharp, the second change, two sharps, and so on. There are two sharp series, a minor one and a major one.
Minor
As each finger is tapped say successively the letters of the scale ABCDEFG:
A B C D E F G A B C D E F G A B C D E F G A B C D E F G A
The Minor key series is produced : AEBFCGDA
Major
The major sharp series is found in the same way except that when you first tap your thumb, don't say A, but say C and continue through the letters of the scale:
C D E F G A B C D E F G A B C D E F G A B C D E F G A B C
The Major key series is produced : CGDAEBFC

Flat Series

The same procedure is used to produce the flat series (both minor and major) but do not tap the little finger. There are two flat series, one minor and one major.
The Minor key series is produced : ADGCFBEA
The Major key series is produced : CFBEADGC

'Sharp' or 'Flat' in the Key Signature name

Whether the word sharp or flat appears in the key signature name depends on its relation to F in each series.
In the sharp series, F and all keys after F have sharp in their name.
In the flat series, only the keys after F have flat in their name.

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