Perfect Fourth

Five semitones. The ratio is four to three, which is simple enough to count as consonant. Turn a perfect fifth upside down and you get a perfect fourth, which is why the two sound like relatives.

Open, slightly hollow, and a little more restless than a fifth. Here Comes the Bride opens on a rising perfect fourth, and that melody is old enough to use freely.

The important part is that a fourth changes job depending on the bass. Play it over the root and it is stable. Play it over the note a fourth below and it wants to resolve.

In sus4 chords, where it replaces the third. In quartal voicings, where chords are stacked in fourths instead of thirds and gravity disappears. In the six-four chord, where a fourth above the bass is unstable enough that classical writing gives it special rules.

With the perfect fifth, its mirror image. Play them back to back over a drone and the difference is obvious in one listen.