Writing all the sharp and flat on a score can make the score hard to read so Key Signatures are used to determine which sharps or flat you want in the entire score.

Sharp Keys

The sharps are written after the clef or time signature if there is one and is written in the following order:

Sharps:    F C G D A E B

The mnemonic phrase used to remember this is "Father Charles Goes Down And Ends Battle".

To find the key of a score you that the last sharp and raise it to the next note, such as G to A, or D to E.
So if the score showed three sharps (F, C, G) again written in the specific order, you take the last sharp G raise it to the next note A so the piece of music or score would be in the Key of A.

Number of Sharps:  1   2  3  4  5   6   7
Key Signature:         G  D A  E  B F# C#

Sharps in Key Signature

Examples of Keys with Sharps

One sharp: F, F to G equals Key of G.
Two sharps F  C, C to D equals Key of D.
Three sharps shown F C G.     G to A equals Key of A.
Four sharps:  F C  D, D to E equals Key of E.
Fives sharps: F C G D A , A to B equals Key of B.
Six sharps: F C G D A  E, E to F equals Key of F#.
Seven sharps: F C G D A E B ,  to C equals Key of C#.
No sharps is the Key of C.

Flat Keys

Flats are also written in a specific order on the staff and that is:

Flats:    B E A D G C F

The mnemonic phrase is then reversed for use in the flat keys: "Battle Ends And Down Goes Charles' Father".

To find the key signature of a flat key you look at the previous flat.
So if three flats are written (B,E,A) look at the next to the last one E and that will be the key signature.
If there is only on flat the key is F.

Flats in Key Signature

Examples of Keys with Flats

One flats: B,  Key of F, must remember.
Two flats B E , go back one flat equals Key of Bb.
Three flats shown B E A, go back one flat equals Key of Eb.
Four flats:  B E A D, go back one flat equals Key of Ab.
Five flats:  B E A D G, go back one flat equals Key of Db.
Six flats:  B E A D G C, go back one flat equals Key of Gb.
Seven flats:  B E A D G C F, go back one flat equals Key of Cb.