The key signature is an arrangement of accidentals at the beginning of a piece indicating the key to the performer. There are 15 key signatures: seven sharp, seven flat and C major which has no sharps or flats.
The order of sharps (#) are arranged as F, C, G, D, A, E, B. There is an easy way to remember the order of sharps which my music teacher told me. This is: Father, Charles, Goes, Down, And, Ends, Battle.
The order of flats (b) is simply the order of sharps in reverse. B, E, A, D, G, C, F. This too can be remembered by reversing the mnemonic to say Battle, Ends, And, Down, Goes, Charles, Father. Silly I know, but it did help a lot with my A-level music.
The major scale rule
In a major scale for example “C”, if you play each note as triads (chords containing three notes), it will be arranged as C major (I), d minor (ii), e minor (iii), F major (IV), G major (V), a minor (vi), and b diminished (vii°). This major, minor and diminished ordering works for every single major key.
Note: I’ve used capital letters to represent major scales; this is the norm in the musical language. The roman numerals represent the number in the sequence from the tonic key which in this case is C. “°” represents a diminished chord.
How to find out if a chord is major, minor, augmented, or diminished
The C major chord will consist of the notes, C, E, and G. The relationship between C and E is a major, and from E to G it is a minor. This major, minor order creates a major chord.
The C minor chord uses C, Eb, and G. C to Eb is minor, and Eb to G is major. The minor major order creates a minor chord.
The C augmented chord is C, E, and G#. C to E is major, E to G# is major. Major to major creates an augmented chord.
C diminished chord is C, Eb, and Gb. C to Eb and Eb to Gb are both minor. Therefore a minor and minor sequence creates a diminished chord.
The relative minor/major
To work out a relative minor of a major key, all you have to do is move down three semitones. In the key of C major, if you move down three semitones you will get to A minor. The relationship between C major and A minor are that they both have the same number of sharps/flats. In this case they both have no sharps or flats in the key signature. A way to remember whether to move up or down three semitones is to remember that “minors live underground” as my music teacher told me. To work out the relative major of a minor key, you simply reverse the process and move three semitones up. Therefore the relative major of A minor, is C major.
Key relationship in music
When writing music, most music will modulate to a different key. In most cases it would modulate to the relative major/minor key. However there are many other keys that music can modulate to. The other two most likely keys that a piece would modulate to are the sub dominant (IV) or the dominant (V), so if you are in the key of C major, the sub dominant would be F major and the dominant is G major.