Originally Posted by: zreynoldsp
Also, a seventh chord is created by fretting the seventh as well as the fifth, not instead of.... it's a four note chord instead of a triad...

He's actually got a point. The 5th of a seventh chord is usually the first to be left out. So it's 1, 3, 7. You'll see this alot in jazz and classical music. If it is found, it's usually the highest tone or it's altered. Like a dom7#5, or dom7b5. In classical music the 5th is always left out (except a few circumstances). It's just what us theory buffs call voice-leading. ;)