In freshman music theory classes, a standard exercise is for the professor to give a simple melody to the class, and the students are assigned to take that melody and add three more voices to it, as if writing for a quartet or a choir. The professor gives the soprano part (melody), and the students are to add the alto, tenor, and bass parts.

A lazy student could simply write the other three parts in a combination of parallel octaves and parallel fifths, thereby creating a somewhat medieval sounding harmony, but not learning much of anything about the harmonic possibilities the melody offers. Hence the rule against parallel octaves and parallel fifths. For music students.

But the fact is that parallel octaves and parallel fifths are not verboten in classical Western European music. Digging through musical scores, one can find all kinds of examples of their use by well-known composers.

"So, why," asks the student sitting in the back row, "can't we use them when we write these exercises?"

The professor answers, "Parallel octaves and parallel fifths create a particular texture of sound because in a parallel octave, the upper octave reinforces the first overtone of the lower octave or fundamental note. And a parallel fifth reinforces the second overtone of the fundamental. The parallel fifth in particular is very strong. And unless you're trying to create that specific effect, it should be avoided. But there are, indeed, times when you want it.

"Let me put it this way:   we begin by teaching you the somewhat strict rules of harmony so that when the time comes when you may want to break these rules, you know why you're doing it."

In the Ian and Sylvia video clip, they use parallel fifths with very good effect, and I'm sure they knew exactly what they were doing. There are other songs in which that affect would be bloody awful!

Don Firth