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What Is a Perfect Fourth?

What Is a Perfect Fourth?

A perfect fourth is an interval of five half steps — E up to A, or A up to D. Guitarists have played millions of them without knowing: most adjacent string pairs in standard tuning are a perfect fourth apart. Same fret, next string up = a fourth (except crossing G→B).

Why "perfect"?

Fourths, fifths, octaves, and unisons are called perfect rather than major or minor because they don't come in bright/dark pairs — they're the acoustically purest intervals after the octave, sitting near the bottom of the harmonic series. A fourth is an upside-down perfect fifth: C up a fourth is F, and C down a fifth is also F.

On the fretboard

  • Adjacent string, same fret (the tuning interval itself).
  • On one string: five frets up.
  • The 5th-fret tuning method works precisely because strings sit a fourth apart.

Sus4 chords get their suspended, unresolved sound by replacing a chord's third with a perfect fourth — the full story is in Sus2 and Sus4 Chords Explained. Ear anchor: the first two notes of "Here Comes the Bride."

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