The Complete Idiot''s Guide to Music Theory

Part 1:
Tones
46
Using Key Signatures
One of the convenient things about assigning a particular key to a piece of
music is that it enables you to designate the appropriate sharps and flats up
front, without having to repeat them every time they occur in the music.
Here’s how it works.
You designate a key by inserting a 
key signature
at the very start of the music,
next to the first clef on the first staff. This key signature indicates the sharps
and flats used in that particular key. Then, when you play through the entire
piece, you automatically sharpen and flatten the appropriate notes.
For example, let’s say you write a song around the F Major scale. The F Major
scale, if you recall, has one flatted note: B-flat. So next to the first clef on the
first staff, you put a flat sign on the B line. Now, when you play that song,
every time you see a B, you actually play B-flat.
The key signature for the key of F—note the flat sign on the B line, indicating the automatic 
B-flat.
The same would apply if you were playing in the key of G, which has one sharp:
F-sharp. You put a sharp sign on the top F line on the first staff; then every
time you see an F, you play an F-sharp.
Major Keys
Just as there are 15 major scales (including three enharmonics), there are 15
major keys; each with its own key signature. The following table shows what
each key of these key signatures looks like, along with its corresponding scale.
The 15 Major Keys
Key
Key Signature and Scale
C Major
C-sharp Major
D-flat Major

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Chapter 4:
Major and Minor Keys
Key
Key Signature and Scale
D Major
E-flat Major
E Major
F Major
F-sharp Major
G-flat Major
G Major
A-flat Major
A Major
B-flat Major
B Major
C-flat Major
47

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