Musical intervals
An interval is the distance between two notes, measured in semitones. It is the basic unit of ear training: every melody is a sequence of intervals, and every harmony is built from combining them.
There are thirteen of them, from the unison to the octave. Each has a character — some sound settled, others ask to resolve — and hearing that character is what lets you name an interval without counting semitones one by one.
Unison P1 0 Perfect consonance Minor second m2 1 Dissonance Major second M2 2 Mild dissonance Minor third m3 3 Imperfect consonance Major third M3 4 Imperfect consonance Perfect fourth P4 5 Perfect consonance Tritone TT 6 Dissonance Perfect fifth P5 7 Perfect consonance Minor sixth m6 8 Imperfect consonance Major sixth M6 9 Imperfect consonance Minor seventh m7 10 Mild dissonance Major seventh M7 11 Dissonance Octave P8 12 Perfect consonance