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Naming Intervals
Builds on Major Scale Construction
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An interval's full name has two parts: a number (how many letter-names it spans, counting both endpoints) and a quality (major, minor, perfect, augmented, or diminished — describing its exact size within that number).
Unisons, 4ths, 5ths, and octaves are called 'perfect' because they stay consonant and structurally stable when inverted; 2nds, 3rds, 6ths, and 7ths come in major or minor versions, with minor always one semitone smaller than major.
⚠️ Common mistake
Counting only the semitone distance and forgetting the letter-count. An augmented 4th and a diminished 5th both span 6 semitones and sound identical on a piano, but are named differently depending on spelling and context — Eardle groups these under the common name 'tritone.'
📝 Summary
Intervals are named by number (letter-span) and quality (major/minor/perfect/augmented/diminished). Perfect intervals are unison, 4th, 5th, and octave; the rest are major or minor.
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