This is the second music theory lesson in this little series, and this one is on seventh chords (chords with four notes). For some reason a lot of people learn the basic triads (three note chords), but they never really get comfortable with the seventh chords. There's no reason not to though. If you understand your triads then learning this is simple; you just learn how to find that fourth note and you're done. They're beautiful chords too, by the way.

If you missed the first theory lesson you can watch it here:
http://youtu.be/5Y01jIorpeA