Name the small distance

Two keys light up: the orange one is the lower. Count the semitones between them — every key counts, black ones included.

One semitone apart is a semitone, two is a whole tone. Three is a minor 3rd, four a major 3rd, seven a perfect 5th.

With practice the counting drops away: a 5th has a width you recognise at a glance, the way you recognise a word without spelling it.

H = 1 · W = 2 · m3 = 3 · M3 = 4 · P5 = 7 semitones

Seven semitones from C to G: a perfect 5th.

Practise this in the trainer — it opens with this lesson the first time the concept comes up.