This is less of a lesson – more of a stream of consicousness – it took me a while to wrap my head around this lesson – so you, the reader are made to suffer too… sorry about that.

The basis of most western music is the C major scale (also called the Ionian scale)

C D E F G A B C
1  2  3  4  5 6  7 8

Major scales follow the pattern of
Full Step | Full Step | half step | Full Step | Full Step | Full Step | half step | Full Step

Chord formula for Major Chords 1, 3, 5 – so if we take the C scale above and extract the 1, 3 and 5 notes we end up with C E G – the notes that make up a standard C Major chord

A major 7 adds in the 7th note – so for the above example the B is also added bringing the chord to C E G B