What does the 7 in C7 mean?
Whenever you see a number after a chord, it refers to the note in that particular scale that you should add to the basic chord. If you know that the C scale (C, D, E, F, G, A, B, C), then C6, for example, is the basic C chord (C, E, G) plus the 6th, which is A. But sevenths are a different matter. If you see a “Cmaj7,” then this would be C, E, G and B, which is the major seventh. A regular “7″ chord means that you want to add a FLATTED seventh note – a major seven which is lowered a half step so that it is one full step below the root. So a “C7″ would be C, E, G and Bb. We go over the formation of these and other chords quite thoroughly in The Power of Three and Building Additions and Suspensions. If you haven’t already done so, you might want to give those a once-over. This answer can be edited and improved by you on the Guitar Noise Wiki. Go to seventh chords and select edit to make improvements.
