Perfect Fifth

Seven semitones. The frequency ratio is three to two, the simplest one after the octave, which is why the fifth sounds so settled. Stack a fifth on a root and you have two of the three notes of a triad, with only the third left to decide the mood.

Open and hollow, with nothing in it that pulls anywhere. Twinkle Twinkle Little Star opens on a rising perfect fifth, and that traditional melody is a safe reference.

The reliable way to learn an interval is to hear it against a drone on the root, not to hunt for a song that starts with it. A song hook fixes the interval in one direction, one register and one rhythm, and the moment it turns up somewhere else you are back to guessing.

In every major and minor triad. In power chords, where it is the only interval present. In the circle of fifths, where twelve of them almost return you to where you started, and the small gap left over is the reason equal temperament exists.

With the perfect fourth, which is its mirror image: flip a fifth upside down and you get a fourth. If you are unsure, listen for which note feels like home.