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Perfect fourth vs perfect fifth: how to tell them apart

The fourth and the fifth are easy to mix up. Hear each rising, falling and together, see how they fill an octave, and learn three ways to tell them apart.

· 6 min read · 2 drills to try, 6 sounds to play

The perfect fourth and the perfect fifth are close relatives, and early in ear training they’re easy to mix up. Many listeners hear both as open and plain next to the sweeter thirds, both are called perfect, and the fifth is only a whole step bigger than the fourth.

This guide shows what sets them apart: how big each one is, how they fit together into an octave, how each sounds rising, falling and played together, and three ways to tell them apart. Every sound on the page is the app’s piano, and there are two short drills.

Two sizes, two simple ratios

A perfect fourth spans 5 semitones, C up to F. A perfect fifth spans 7, C up to G. They share the quality perfect, which Open Music Theory gives only to unisons, fourths, fifths and octaves.

Both have very simple frequency ratios. In a fifth the upper note vibrates three times for every two vibrations of the lower note, 3:2; in a fourth it vibrates four times for every three, 4:3. Both also sit at the bottom of the harmonic series, the row of quieter frequencies inside every piano note: the fifth lies between its second and third harmonics, the fourth between the third and fourth.

Perfect fourth 4:3

C4 up to F4: a perfect fourth, 5 semitones.

A perfect fourth and a perfect fifth, both rising from C4. The fourth lands on F, the fifth on G. The fourth’s figure, 4:3, is a little busier than the fifth’s.

Turn a fifth over and you get a fourth

The two are inversions of each other. Move the lower note of an interval up an octave and you get its inversion: C up to G is a fifth, and G up to the next C is a fourth. Open Music Theory puts it as a rule: fourths invert to fifths, fifths invert to fourths, and perfect intervals stay perfect.

So a fifth and a fourth stacked on top of each other fill an octave exactly. In semitones, 7 plus 5 make 12. The ratios multiply the same way: 3:2 times 4:3 is the octave’s 2:1. You may know the stack already. The brass fanfare that opens Richard Strauss’s Also sprach Zarathustra climbs C, G and C again: a fifth, then a fourth.

The pairing even shows in how a piano is tuned. Equal temperament keeps every octave exact and nudges the other intervals off their just ratios, so on a piano the fifth is 2 cents narrower than 3:2 and the fourth is 2 cents wider than 4:3: the same small amount in opposite directions, because together they still have to make a whole octave.

The fourth, the fifth and the octave they make together. Press a figure to hear its interval with both notes at once.

How each one sounds

Descriptions are personal, so lean on tunes you already know. They also reveal a pattern.

Rising. Many familiar tunes open with a fourth from sol up to do, from the fifth note of the scale up to the home note: “Amazing Grace”, “Auld Lang Syne” and “O Christmas Tree” all start this way. The Bridal Chorus from Wagner’s Lohengrin, better known as “Here Comes the Bride”, opens with a rising fourth too. “Twinkle, Twinkle, Little Star” shows a rising fifth going the other way, from do up to sol: it begins do, do, sol, sol. In these tunes a rising fourth arrives home, and a rising fifth sets out from it.

Falling. Turn them around and the roles swap. A falling fifth from sol to do lands home: the Minuet in G from the 1725 Notebook for Anna Magdalena Bach, written by Christian Petzold, opens with one. A falling fourth from do to sol steps away: Mozart’s Eine kleine Nachtmusik opens with one, and “O Come, All Ye Faithful” has one between its second and third notes.

Perfect fourth 4:3

G4 down to D4: a perfect fourth, 5 semitones.

Both falling from G4. The fourth stops on D4, the move from the second to the third note of “O Come, All Ye Faithful” in G major. The fifth goes on down to C4.

Together. Played at once, the fifth makes a bare or open fifth, a chord with no third, which on its own doesn’t say major or minor. Beethoven’s Ninth Symphony begins with open fifths on A and E, and rock guitarists call the same sound a power chord. Violins are tuned in fifths, G3, D4, A4 and E5, and when two neighboring strings are tuned to exactly 3:2, the result is smooth and consonant.

The fourth is the ambiguous one. To the ear it’s a consonance, and guitars are tuned in it: all but one pair of a guitar’s neighboring strings are a fourth apart, and on a bass guitar every pair is. But in common-practice harmony it counts as a dissonance whenever it sits above the bass, and Open Music Theory lists it with the harmonically dissonant intervals, though it’s consonant as a melodic leap. Its standing has moved over the centuries: in the 13th century the fourth and the fifth were ranked together as consonances, and only in the 15th century did the fourth come to be treated as a dissonance on its own. Many listeners still hear a fourth played together as less settled than a fifth.

Paul Hindemith and David Cope both describe one way to think about this, the interval’s root. Find the interval in the harmonic series; the note that is an octave of the fundamental is its root. In a fifth that’s the lower note, and in a fourth it’s the upper one. Many listeners hear it that way too: a fifth resting on its bottom note, a fourth leaning on its top note.

Perfect fourth 4:3

C4 and F4 together: a perfect fourth, 5 semitones.

C4 with F4, then C4 with G4, both notes at once. Listen for the note that sounds like the foundation: in the fifth many listeners hear the C, in the fourth the F.

Fourth or fifth, together?

6 sounds

    Three ways to tell them apart

    1. Sing up the scale. Take the lower note as do and sing upward: do, re, mi, fa is a fourth, and one more step, sol, makes a fifth. That’s how intervals get their names, counting the notes from one. It’s slow, so use it to check an answer rather than to find one.
    2. Finish the octave. After you hear the interval, imagine the note an octave above the lower one. After a fifth, the gap left to the octave is the smaller one, a fourth; after a fourth, the gap left is a fifth. You’re comparing the two halves of an octave instead of judging one interval on its own.
    3. Listen for home. In the tunes above, rising fourths and falling fifths land on do, while rising fifths and falling fourths leave it. That’s common, not a rule: a fourth can start on any note of a scale. Let the feeling point you to an answer, then check it.

    If you like learning with songs, “Here Comes the Bride” and “Twinkle, Twinkle” make a good first pair, one for each interval. Songs to remember intervals has tunes for the other intervals too, and when to stop needing them.

    Practice them side by side

    Now try the pair with one note after the other. The drill plays a rising fourth or a rising fifth from a different note each time. If you miss one, press “Hear both” to compare them on the same note.

    Fourth or fifth?

    6 sounds

      When the rising pair feels easy, practice the falling forms the same way. Sing a note, then sing down to the note a fourth below and the note a fifth below, and say which is which as you go. Singing both from the same starting note puts the two sizes side by side.

      In Earstep the fourth joins at level 3 of Rising intervals, “Thirds, fourth and fifth”, next to the fifth and both thirds, and the falling and together forms have paths of their own. Come back to the pair for a few minutes on several days rather than all at once.

      Sources