Seconds and thirds

An interval is the distance between two notes, counted inclusively: from C4 to D4 is a second; from C4 to E4 is a third. What matters is that each interval has a fixed shape on the staff and can be read without counting.

• A second: one note on a line, the other in the space right next to it. They touch, almost overlapping.
• A third: both on lines or both in spaces, skipping one position.

This is the same "step or skip" distinction as unit 1, now under its proper name. From here on intervals are called by their number.