Parallel Intervals: Definition & Types

Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

There are many rules to writing good four-part music, and in this lesson we'll look at a rule to help keep all four lines distinct. Let's check out parallel intervals and see when they are, and are not, appropriate.

Rules for Four-Part Music

If you were going to compose a painting and only let yourself use four colors, you'd want to make sure that those colors worked together, right? Each needs to be distinct, but still complement the others.

That same idea applies to writing four-part music. You've got the four voices or lines of soprano, alto, tenor, and bass. Each line needs to have its own purpose and distinction, but you also want them to work well together. Luckily, there are a number of rules to help us with four-part writing. One of the basic rules is to understand motion, or the ways that each line moves between notes. It's part of how we create a masterpiece instead of a mess.

Parallel Intervals

Imagine that you have two notes being played at the same time, a C and a G. There's the start of two voices or lines of music right there. Each is going to develop independently. So, let's see what happens. In the next measure, the bottom voice moves to a D, and the top voice moves to an A. That's significant.

To understand why, we need to examine the relationships between each set of notes. In the scale of C Major (C, D, E, F, G, A, B, C), the C and G are five notes apart, so they create an interval of a fifth. That's a very stable relationship between these two voices. However, D and A are also five notes apart. So, despite the fact that both voices developed, they did so in the exact same way. The interval between them is still the same.

This is known as a parallel interval, when two voices move in a way that maintains the relationship between them. This isn't just something that happens with fifths. You can have parallel thirds or parallel sevenths; as long as the interval between two voices is maintained, we're experiencing parallel motion.

Parallel intervals, as used by Claude Debussy in The Sunken Cathedral

Objectionable Parallels

When composing music, parallel development is something you want to keep an eye out for (which can be hard to do since there are four independent voices that each have to be compared to the others). Parallel intervals are acceptable in four-part music, but only certain ones. Parallel thirds and fourths show up every now and again, and sometimes you'll even see parallel seconds, sixths, and sevenths.

What you want to avoid, however, are parallel fifths and parallel octaves (also called consecutive fifths and consecutive octaves). This means that if you start with C and G, the next notes should not be D/A, E/B, F/C, G/D, A/ E, B/G, or the C/G an octave higher. Since parallel fifths and parallel octaves are against the rules in four-part music, we call them objectionable parallels.

An extreme case of parallel fifths

But what makes them so objectionable? Why is it technically OK to use parallel thirds, but not parallel fifths? The reason is that four-part music is written to have, well, four parts. Each line needs to be independent. An interval of a fifth is extremely stable and harmonic, and an octave is just the same note played higher on the scale. For one beat, that's fine to do, but when both voices develop the same way, they start to lose their individual integrity.

Parallel intervals obscure the four distinct parts of four-part music because at least two of the lines are too similar to each other. They're not easily distinguishable, but instead sound like a harmony of a single line.

Parallels by Contrary Motion

While popular music and folk music actually make use of parallel fifths pretty often, composers of four-part music are advised to steer clear of any objectionable parallels. This can get pretty tricky and is one of the challenges in writing four-part music. You've got to make sure that none of the lines are maintaining the same intervals as they develop.

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