These seven chords are called the diatonic chords to C major. (The name .) The diatonic chords are good to know. You can play them in any order and any combination, and the C major scale sounds terrific over all of them.
The roman numerals next to each chord refer to the scale degree the chord is based on. G is the V chord in C major because G is the fifth note in the C major scale. These numbers can be a good shorthand. You’ll see references to chord progressions like I-IV-V, which means, in C, play C, F, G. Another common progression is I-vi-ii-V — that’s C, Am, Dm, G.
The first note in each chord is called the root. The next one is the third (makes sense, you skipped the second.) After that is the fifth. If you add another note to each one, you get seventh chords.
Cmaj7: C E G B
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