I think one of the more general approach can be broken down into chords and scales.
The way I see, I like to hinge everything back to the major scale:
W W H W W W H
E.g. C Major Scale (key of C major) is made up of the notes:
C D E F G A B
Chromatically speaking, there are in total 12 notes, i.e.:
C C# D D# E F F# G G# A A# B
Here, you'll realize that in the key of C major, C to D is 1
Whole interval, while E to F is
Half an interval apart...
So, by the same token of the major scale formula, you
can find out all the notes in every other keys.
E.g. In the key of G major, the notes are:
W W H W W W H
G A B C D E F# G
Application: By understanding how to derive the family notes in any key, it allows me to quickly derive all the family chords and the notes in any key. This is especially useful when i am required to transpose a song / music piece from one key to another on the spot during band practice.
Anyhow, here is only an example of my understanding in music theory. if you would like to know more, feel free to contact me.
mobile: 91149094
P.s. If you are interested to know how chords are formed, there is theory behind chordal construction as well. One of the greatest skill I picked up as a guitarist is to know how to form chords in different voicings... Once I know that, basically I have no more use for those "guitar chords encyclopedia" since I can even form my own chords from scratch...
Cheers,
Han Yew