An interval is the distance between two notes, measured in semitones: 12 of them span every melody from a minor 2nd to an octave.
An interval is the distance between two notes: how far the pitch jumps. Small jumps sound like steps; big jumps sound like leaps.
Every melody you know is a chain of intervals. The first two notes of Here Comes the Bride are one specific interval (a perfect 4th) no matter what key it is sung in, which is why interval hearing lets you play tunes in any key.
Intervals are measured in semitones, the smallest step in Western music. There are 12 simple intervals: minor 2nd (1 semitone) through the octave (12), each with its own recognizable character.
The names combine a number (2nd, 3rd, 4th...) counting letter names, and a quality (major, minor, perfect). The 6-semitone tritone sits exactly in the middle of the octave and drives much of harmony’s tension.
Interval recognition is the core of relative pitch: name the distances and you can name a melody’s skeleton without perfect pitch.
Number comes from counting inclusive letter steps (C to E is a 3rd: C, D, E), quality from the exact semitone count (C to E, 4 semitones, is major; C to E-flat, 3, is minor). The perfect intervals (unison, 4th, 5th, octave) keep one quality name for historical and acoustic reasons: their frequency ratios are the simplest.
Beyond the octave, intervals become compound: a 9th is an octave plus a 2nd. Ear training treats them as their simple equivalents plus register.
Trainable order that works: octave, perfect 5th, major 3rd first (maximally distinct), then fill in, then descending forms, which most courses skip and real music demands.