What is the principle or the rules in finding the key signature?

Answer 1

TO FIND THE KEY FROM THE KEY SIGNATURE

If the key signature has sharps:

The major key is the note above the last sharp in the key signature.

The minor key is the note below the last sharp in the key signature.

Ex: key signature with sharps f-c-g is A major or F-sharp major

If the key signature has flats:

The major key is the note of the next to last flat in the key signature. If there is only one flat, it is F major.

The minor key is the note TWO notes above the last flat in the key signature.

Ex: key signature with the flats b-e-a-d-g is D-flat major or B-flat minor

If the key signature has no flats or sharps:

The major key is C.

The minor key is A.

TO CREATE THE KEY SIGNATURE FROM THE KEY

The sharps always go in this order: F-C-G-D-A-E-B-C (circle of fifths)

If you are creating the key signature for a major key, keep writing sharps until the last sharp is the note below the key you need.

If you are creating the key signature for a minor key, keep writing sharps until the last sharp is the note above the key you need.

Ex: The key signature for B major and G-sharp minor is f-c-g-d-a.

The flats always go in this order: B-E-A-D-G-C-F (circle of fourths)

If you are creating the key signature for a major key, keep writing flats until the next-to-last flat is the key you need.

If you are creating the key signature for a minor key, keep writing flats until the last flat is TWO notes below the key you need.

Ex: The key signature for E-flat major and C minor is b-e-a.

The key signature for C major and A minor has no flats or sharps.

The key signature for F major and d minor has ONE flat (B-flat).