Organum (850c - 1100c)

• one of the earliest forms of counterpoint (multiple melodies)
  example: Pachebel's Canon in D
• he uses 3rds and 5th intervals between the two melodies (stable, major chord intervals)

Baroque (c.1600 - 1750)

• chords explicitly defined (as opposed to emerging out of overlapping melodies)

Classical (c. 1750 - 1820)

• chords become primary.