How do parallel thirds create a tritone?
Question: I'm practicing polyphony with counterpointer and it says that parallel thirds will create a tritone. Could you explain? - Ngo

Answer: It's always a pleasure to hear from someone who is studying counterpoint - an esoteric art.

I hope you won't mind if I review first what a tritone is, just in case that's part of the question. "Tritone" is a name for the interval of an augmented 4th, because it is formed of 3 whole tones added together. It traditionally was regarded as a difficult interval: hard to sing and hard to get in tune; it was thought to have a harsh sound. The same was true of its inversion, the diminished fifth. The ideal 16th century counterpoint tries to avoid leaping by a tritone, or even outlining one in melody.

If two voices separated by a major third both move in the same direction by a whole step, the lower and upper notes of the first and second intervals will form a tritone. That's the origin of the "rule" about parallel major thirds.

Fortunately it doesn't come up that often: in every mode and in the major scale there is only one place where this can happen naturally. In the major scale it's between the 4th and 5th degrees:

Sometimes students will be heard to complain that these rules are too restrictive. As I like to point out and I expect have repeated way too many times, the "rules" aren't like federal law; you're not going to do time if you break them; they are a recipe for obtaining a certain style. You can write differently if you want to, but then the music won't be in the 16th-century style that counterpoint students strive to emulate. Following a particular style is a good discipline that gains you skill and understanding of the material. After that you do whatever pleases you.
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