Parallel Intervals: Definition & Types

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  • 0:04 Four-Part Music: Rules
  • 0:47 Parallel Intervals
  • 2:01 Objectionable Parallels
  • 3:42 Contrary Motion
  • 5:08 Lesson Summary
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Lesson Transcript
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; in this lesson we'll look at a rule that helps keep all four lines distinct. Let's check out parallel intervals and see when they are, and are not, appropriate.

Four-Part Music: Rules

If you were going to compose a painting and limited yourself to the use of four colors, you'd want to be sure that those colors worked well together, right? Each color would need to be distinct, while still complementing the others.

That same idea applies to writing four-part music. You've got the four lines or voices of soprano, alto, tenor, and bass. Each line not only needs to be distinct and have its own purpose, but also work well together. Luckily, there are a number of rules that can help to guide four-part writing. One of the basic rules is that of motion, or the ways that each line moves between the notes. It's how we create a masterpiece instead of a mess.

Parallel Intervals

Imagine that you have two notes being played at the same time: C and G. That's the start of two lines of music or voices right there. Each line is going to develop independently. So let's see what happens. In the next measure, the bottom voice moves to a D, while 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, resulting in an interval of a fifth. Therefore, these two voices share a very stable relationship. 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 something that just happens with fifths. You can have parallel thirds or parallel sevenths. As long as we maintain the interval between two voices, we have parallel motion.

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 each one of which has 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. 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 and A, E and B, F and C, G and D, A and E, B and G, or the C and G an octave higher. Since parallel fifths and parallel octaves break the rules in four-part music, we call them objectionable parallels.

But what makes them so objectionable? Why is it technically okay 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, while 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.

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