Unisons, Octaves, Fourths, and Fifths in Music Theory

By Michael Pilhofer, Holly Day

Familiarizing yourself with the elements of music theory can improve your execution. Unisons, octaves, fourths, and fifths share the same characteristics in that they all use the terms perfect, augmented, or diminished to identify their quality.

Perfect unisons

A perfect melodic unison is possibly the easiest move you can make on an instrument (except for a rest, of course). You just press a key, pluck, or blow the same note twice. You can play unisons on most stringed instruments because the same note occurs more than once on these instruments, such as on the guitar (the fifth fret on the low E string is the same as the open A string, for example).

In music written for multiple instruments, a perfect harmonic unison occurs when two (or more) people play exactly the same note, in the exact same manner, on two different instruments.

Augmented unisons

To make a perfect unison augmented, you add one half step between the notes. You can alter either of the notes in the pair to increase the distance between the notes by a half step.

The interval from B flat to B is called an augmented unison (or augmented prime) — unison because the note names are the same (both Bs) and augmented because the interval is one half step greater than a perfect unison.

A diminished unison doesn’t exist, because no matter how you change the unisons with accidentals, you’re still adding half steps to the total interval.

Octaves

When you have two notes with an interval quantity of eight lines and spaces, you have an octave. A perfect octave is a lot like a perfect unison in that the same note (on a piano, it would be the same white or black key on the keyboard) is being played. The only difference is that the two notes are separated by 12 half steps, including the starting note, either above or below the starting point.

The perfect melodic octave has 12 half steps between the notes.

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To make a perfect octave augmented, you increase the distance between the notes by one more half step. Here is an augmented octave from E to E sharp. It was augmented by raising the top note a half step so that 13 half steps come between the first note and the last. You could also make an augmented octave by lowering the bottom E note a half step to E flat.

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To make the same octave diminished, you decrease the distance between the notes by one half step. This example lowers the top note a half step so that only 11 half steps come between the first note and the last. You could also raise the bottom note by a half step to make another diminished octave.

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Fourths

Fourths are pairs of notes separated by four lines and spaces. All fourths are perfect in quality, containing five half steps between notes — except for the fourth from F natural to B natural, which contains six half steps (making it an augmented fourth). Compare the note pairs on the keyboard to see this.

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Here, you see the connection between each fourth on a keyboard. Note that unlike the rest, the F natural and B natural require six half steps.

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Because augmented fourths are a half step larger than perfect fourths, you can create a perfect fourth between the notes F natural and B natural by either raising the bottom note to F sharp or lowering the top note to B flat.

If the natural fourth is perfect, adding the same accidental (either a sharp or a flat) to both notes doesn’t change the interval’s quality. It stays a perfect fourth. The same number of half steps (five) occurs between D natural and G natural that occurs between D sharp and G sharp, or D flat and G flat.

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If one note changes but the other doesn’t, the quality of the interval does change.

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Fifths

Fifths are pairs of notes separated by five lines and spaces. Fifths are pretty easy to recognize in notation, because they’re two notes that are exactly two lines or two spaces apart.

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All fifths are perfect fifths, meaning that the interval contains seven half steps. However, as you may have guessed, the interval between B to F is a diminished fifth, which turns out to have the same sound as an augmented fourth. Only six half steps occur between those two notes whether you’re going from F natural to B natural or B natural to F natural.

You can create a perfect fifth between F natural and B natural by adding one more half step — either by turning the B natural to a B flat or by raising the Fnatural to an F sharp. This time, because the notes are flipped around from the order they appeared in the ­interval of fourths, both changes increase the size of the interval.

Again, just as in a perfect fourth, if a fifth is perfect (every case except F natural and B natural), adding the same accidentals to both notes in the interval doesn’t change its quality. And, as with fourths, if only one of the notes is altered with an accidental, the quality does change.