Studying or practicing Ear Training?  If you're a musician, you should be.  You'll find my Palm software Ear Trainer invaluable.  Also, I teach guitar lessons in Orange County, CA.  (See OCGuitarLessons.com)

 
A triad is a group of three different pitch classes, which is the bare minimum to meet the definition of a chord.  The triads are the most basic chord-type, and are prevalent in pop and folk music.  The tetrads (four-note chords) or "sevenths" (so-called because the fourth note is an interval of a "seventh" above the root) also occasionally occur in pop and folk, but have been especially prevalent in jazz.
 

 
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