Every piece of music is written in one key or another, such as the key of Bb or the key of E or whatever. The key signature at the beginning of a score tells what sharps or flats are being used in the song, and therefore what scale the song is based on.
A key signature is included in each and every musical piece. This information is written right after the clef (shown on the staff) and is explained with symbols stating the number of flats (b) and the number of sharps (#). As you look over the beginning of the lines of music you will recognize a grouping of flats and sharps, which are not ever used simultaneously. They will appear separately on a space or on a line of the music staff and will be placed with the specific notes that will be affected by them. For example, if the music is to played using an F#, the sharp symbol, the #, will be placed on the top line of the staff. This will tell the person that is reading the music that each time an F is played, no matter if it is on top of the staff, in the first space, or below or able the staff (indicated by the ledger lines), this note should be placed as F#.
If a note that is dictated as sharp or flat by the key signature is to be played as natural, an accidental is placed before the note. For example, if an F (natural) is to be played in the key of G, an accidental has to be placed before it so it is not played as an F#.
The main purpose of the key signature is to limit the number of flats or sharps noted in the music. In other words, rather than placing a # by the note F every time it occurs in the music, it’s much easier to indicate to the musician that all Fs are to be played as F#s. Without key signatures, written music would be cluttered with sharps and flats making it very difficult to read.
Here are some common key signatures and the notes they affect:
Key of C: No flats or sharps
Key of G: One designated sharp (F#)
Key of D: Two designated sharps (C# and F#)
Key of A: Three designated sharps (C#, G# and F#)
Key of E: Four designated sharps (D#, F#, C# and G#)
Key of F: One designated flat (Bb)
Key of Bb: Two designated flats (Eb and Bb)
Key of Eb: Three designated flats (Ab, Bb, and Eb)
There is also a relative minor assigned to each key. While it is true that in nearly all ways the major and the relative minor that coincides with it are similar (they use the same scale even though they begin in different places), they are not considered the same. The note that can be found a minor third down from the major (key) is called the relative minor. You can think of it in another way by remembering the sixth note in the major scale is the relative minor. As an example, The C major scale has a sixth note of A so an A minor is the relative minor to C. It is very common to use the relative minor of the key and if you gain a good understanding of them you will find that you have a better understanding of the chord progressions in the song.
Below are some specific keys and what their relative minors (keys) are:
A minor is the relative minor for C.
B minor is the relative minor for D.
F# minor is the relative monitor for A.
G minor is the relative minor for Bb.
An experienced musician playing in a non-formal setting (not using written music) simply needs to know the key to a particular song to be able to play the chords and scales (melodies) for that song.
Lastly, a song being written in more keys than one is fairly common and the key changes in the middle somewhere. The newer musicians may have difficulties when the keys change like this.