The distance between two notes. Instead of measuring in meters, we measure in semitones. Let’s say we want to find the interval between B and G. Forget about music, just take the alphabet and start counting from B (not from C!). So we have: B, C, D, E, F and G. That’s 1, 2, 3, 4, 5 and 6. So B and G are a sixth apart. If we are to clarify what kind of sixth we are talking about, we would distinguish whether the two notes are closer together or further apart by saying whether they are minor or major, diminished or augmented. More or less, that’s the essence.