I'm taking a look at chord construction, and I'm trying to link it to the few chords I can still remember from when I used to play the guitar. Now, a Cmaj chord is made up with C E G. But the way I learnt to play it (powerchord style) you can't hit the open E (lowest one), only C E G C E (from low to high-sounding). But it's a part of the chord, so why can't I just play it? I realise this is more guitar related , but is there some essential piece of theory I'm missing out on? (eg a limitation on the number of times you use a certain note in a chord or something).