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Interval quality (major, minor, perfect, augmented, diminished) is determined by counting the number of semitones between two pitches. For example, a major third contains 4 semitones, a minor third contains 3 semitones, and a perfect fifth contains 7 semitones. Combining letter-name counting with semitone counting allows you to identify any interval precisely.
Count semitones between intervals on a keyboard or with a chromatic scale. Create a reference chart of common intervals and their semitone counts. Practice identifying intervals both visually on the staff and by ear.
Students often confuse the interval name (determined by counting letters) with interval quality (determined by semitones). They may also assume all major intervals are 'better' than minor intervals, not understanding they simply have different harmonic functions.
Identifying an interval precisely requires two separate operations, and confusing them is the most common beginner error. You already know how to count letter names to get the interval number — C to E is a third (C, D, E — three letters). But "third" alone is incomplete: a minor third and a major third are different intervals with very different sounds and functions. The interval quality is determined by counting semitones — the actual pitch distance, not just the letter distance. This second count is what this topic teaches.
Start with the keyboard as a reference tool. Every adjacent key (including black keys) is one semitone apart. From C up to E♭ is 3 semitones; from C up to E♮ is 4 semitones. Both span three letter names (C–D–E), so both are thirds. But 3 semitones gives a minor third, and 4 semitones gives a major third. The interval number (third) stays the same; the quality changes with the semitone count. The key reference facts to internalize are: unison = 0 semitones; major second = 2; minor third = 3; major third = 4; perfect fourth = 5; tritone (augmented fourth or diminished fifth) = 6; perfect fifth = 7; minor sixth = 8; major sixth = 9; minor seventh = 10; major seventh = 11; perfect octave = 12.
Notice that the words "major" and "minor" apply to seconds, thirds, sixths, and sevenths — intervals that come in two natural versions. The word "perfect" applies to unisons, fourths, fifths, and octaves — intervals that have a single natural version in the diatonic scale, highly consonant and stable. If you expand a perfect interval by a semitone, it becomes augmented; if you compress it, it becomes diminished. Major intervals shrunken by a semitone become minor; minor intervals shrunken by a semitone become diminished; major intervals expanded by a semitone become augmented.
The two-step process is: (1) count letter names to determine the interval number; (2) count semitones to determine the quality. For example: C to A♭. Step 1: C, D, E, F, G, A — six letters, so it's a sixth. Step 2: C to A is 9 semitones (a major sixth), but A♭ is one semitone lower than A, so C to A♭ is 8 semitones — a minor sixth. Neither step alone is sufficient. Letter counting without semitone counting leaves quality undetermined. Semitone counting without letter counting can misidentify the interval number (C to D♯ is 3 semitones, same as C to E♭ — but one is a second and one is a third).
Building this skill pays off immediately when you begin constructing triads: a major triad is a major third (4 semitones) plus a perfect fifth (7 semitones) above the root; a minor triad is a minor third (3 semitones) plus a perfect fifth. Your ability to hear and name intervals is also the foundation of all ear training — recognizing that the opening of "Happy Birthday" begins with a major second, or that a perfect fifth has a particular open, stable resonance, gives you perceptual anchors for everything from chord identification to melodic dictation.
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