Read the interval

Two notes, one after the other. How far apart are they?

The shape tells you most of it. A 3rd skips exactly one position, so both notes are on lines, or both are in spaces. A 5th skips three, so again line to line, or space to space, but wider.

The shape cannot tell a minor 3rd from a major 3rd: they sit on the same two positions. Only the accidentals separate them. Read the shape for the size, then the signs for the quality.

line to line, or space to space: an odd interval

E up to G: line to line, so a 3rd. Three semitones, so a minor one.

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