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Octave: Definition, Function & Examples

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  • 0:04 Definition of an Octave
  • 1:13 Frequency Relationship
  • 2:07 Octave Equivalency
  • 3:14 Octaves in Music
  • 4:12 Lesson Summary
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Lesson Transcript
Instructor: Greg Simon

Greg is a composer and jazz trumpeter. He has a doctorate from the University of Michigan and has taught college and high school music.

This lesson teaches you about the musical interval of an octave and how composers use it, and introduces you to some examples of its use in different types of music.

Definition of an Octave

An octave is a type of musical interval, or the measure of the distance between two notes. Specifically, an octave is the distance between one note and another note with the same letter name.

Take a look at the piano keyboard pictured here. While we'll use a piano for the examples in this lesson, octaves can be found on any instrument.

An octave on a keyboard (original image by Scott Detweiler)

Count from the note labeled C to the next note labeled C - that's an octave. You might have noticed that the distance spans eight of the keyboard's white keys (including the two Cs themselves). The 'oct-' in octave comes from a Latin prefix meaning 'eight.' That's where the octave gets its name!

There are other ways to think of an octave as well. In addition to being the distance between two similarly named notes, an octave spans twelve half steps: on the keyboard, any two adjacent keys are a half step apart. Check the keyboard again, not counting the first C. You'll discover that the journey from one to the other takes us across twelve individual keys. Since two half steps combine to a single whole step (the distance between two adjacent white keys in most places on the piano), we can also say the octave spans six whole steps. While composers have manipulated all of these divisions in different ways, let's concentrate for now on how the octave itself has been used.

Frequency Relationship

Each musical note represents a different sonic frequency, or the speed at which the sound vibrates. Two notes separated by an octave have a 2:1 frequency ratio; that is, the higher note has a frequency twice as high as the lower note. This relationship means that octaves can often be perceived as two of the same note, since their frequencies are so closely related.

Because of an octave's frequency relationship, it's said to be the most consonant, or pleasant-sounding interval. Consonance also indicates stability; that is, the octave does not have a tendency to move or resolve to any other type of interval. Because the octave is consonant and stable, composers since the 15th century have treated it as a point of resolution, or a destination for other intervals and melodic lines to resolve to. Octaves often occur at the ends of pieces, especially pieces from the Renaissance and Baroque eras, which rely on counterpoint, or the interaction of two or more melodies.

Octave Equivalency

When composing melodies, composers often manipulate the property of octave equivalency, or the idea that, just as notes an octave apart share the same letter name, they share the same musical properties. Put differently, since all notes separated by an octave are the same, they can all be treated as having the same musical function.

Octave Equivalency

In this diagram, the note an octave away from C is also called C, and the note an octave away from G-flat is also G-flat. In fact, you'll notice that both members of the octave are surrounded by the same array of black and white keys and that their places on the piano are functionally the same. Both of these pairs of notes share musical function with their partners an octave (or multiple octaves) away.

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