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Degree decides what an interval is *called*; interval size decides how *wide* it actually is. Both rulers are needed at once: degree counts across seven letters, size counts semitones on a keyboard. One degree can have different sizes — and the flip side is that one size can land on different degrees.
The semitone is the smallest step
Twelve-tone equal temperament cuts the octave into 12 equal parts; one part is a semitone, and two semitones make a whole tone. The semitone is the smallest unit in the system — nothing smaller gets its own note name (players can bend a pitch by less, but that is intonation, not a new name).
The keyboard makes it obvious:
| Start | One semitone up | One whole tone up |
|---|---|---|
| C | C♯ (black key) | D |
| E | F (white key) | F♯ |
| B | C (white key) | C♯ |
Note that E → F and B → C are semitones without any black key. This single fact shapes the scale: a major scale sounds like a major scale because its two semitones always fall in the same two places.
The size table: the second ruler
| Interval | Semitones | Interval | Semitones |
|---|---|---|---|
| perfect unison | 0 | perfect fifth | 7 |
| minor second | 1 | minor sixth | 8 |
| major second | 2 | major sixth | 9 |
| minor third | 3 | minor seventh | 10 |
| major third | 4 | major seventh | 11 |
| perfect fourth | 5 | perfect octave | 12 |
| augmented fourth / diminished fifth | 6 |
This table is what ear training actually runs on: when you identify an interval, the ear is comparing semitones. The written name (major third or diminished fourth) still needs the degree to settle — which is exactly where enharmonic equivalents come from: 6 semitones can be written as an augmented fourth or a diminished fifth.
Diagram: where semitones hide between white keys
Listen: semitone against whole tone
Hear the minor second (the narrowest), then the major second, then a perfect unison as a control. The difference is only a few tens of hertz, yet it is the deepest dividing line in music theory.
Common misconceptions
- "A semitone means black key to white key." E–F and B–C are white to white and are still semitones. Black keys are invented names, not a definition of width.
- "Same number of semitones means the same interval." 6 semitones can be an augmented fourth or a diminished fifth — same width, different spelling and function.
- "A whole tone is twice as far, so it sounds twice as far." Perception is not a linear multiple; the heard gap between them is far smaller than the ratio suggests.
- "A semitone is the smallest possible pitch difference." It is the smallest difference between note names. Players can deviate by less, but those heights have no names of their own.
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Listen in the library
All tracks from midicn-lib — inline badge is the license tier (C1 commercial / C2 non-commercial / C3 study only).
Sources
- 半音为最小音高单位、十二平均律下八度 = 12 半音，属乐理通则
- 各音程的半音数表（一至八度）为通行表述，本文行文为原创
Used for fact-checking only; all prose is original.