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1.
1. A subject that 75% of "musicians" out there have never touched.
2. An excuse the ignorant make to put down artists that try to push the boundaries of contemporary music. All the while these same ignoramuses believe poor technique and slop equate with good music just because everybody else thinks so. These people also have never bothered to actually listen to the music they loathe so much.
3. A set of (flexible) rules that dictate the manner in which music functions.
In most introductory music theory courses, students learn about the intervallic structure of the major scale and how it is harmonized.
by Brian February 26, 2004
 
2.
Musical theory is vital knowledge for anyone wishing to compose music involving more than half a dozen instruments. It is recommended knowledge for anyone wishing to arrange music for more than half a dozen instruments. Anyone who writes music for more than about half a dozen instruments without knowledge of musical theory is inevitably either writing an arrangement of a simpler piece or composing modern music.

Serving suggestion: Musical theory best served with at least 2 years experience with actual musical instrument.
John spent two and a half years at the Vienna Conservatorium of Music learning musical theory.
by Bob Hu January 05, 2004