Fifth
A fifth is an interval of five letter names, such as C–G. A perfect fifth is seven half steps; it is the distance between the bottom and top notes of a triad.
From C to G there are five letter names, both ends included: C, D, E, F, G. That is a fifth. On the staff it is easy to spot: both notes on lines, or both in spaces, with one line skipped between them.
From C to G is seven half steps: a perfect fifth. D–A, E–B, F–C, G–D and A–E are the same size. The exception among the white keys is B–F: only six half steps, a diminished fifth. Fifth and fourth add up to an octave: C–G is a fifth, G–C a fourth.
The outer notes of a triad make a fifth: in C–E–G, C and G. It is the same in a major and a minor triad. The fifth degree of the scale is called the dominant, and its chord stands a fifth above the tonic. “Twinkle, Twinkle, Little Star” (the theme of Mozart’s variations) opens with a fifth up: C–C–G–G.
What it sounds like
A fifth sounds open, wide and a little hollow – like a space waiting to be filled. Many tunes begin with it.
Where it is taught: the studio’s interactive textbook, chapter 15, “Intervals: unison to octave”.