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A major triad = root + major third (4 semitones) + minor third (3 semitones); minor triad reverses the third order

Principle L1Foundations A · Music theory & musicianship

A triad is built from two stacked thirds. A major triad stacks a major third (4 semitones) above the root, then a minor third (3 semitones) above that, placing the fifth 7 semitones above the root. A minor triad stacks a minor third (3 semitones) first, then a major third (4 semitones), also totalling 7 semitones to the fifth. The key difference: the third (middle note) determines major vs. minor quality. Major sounds bright; minor sounds darker and moodier. The semitone rule lets you build any triad from any root on the chromatic scale without knowing the key.

Examples

C major: C-E-G (C+4 semitones=E, E+3 semitones=G). C minor: C-Eb-G (C+3=Eb, Eb+4=G).

Assessment

Build Bb major and G minor from scratch using the semitone rule. Identify which note changes between C major and C minor, and by how much.

Prerequisites — understand these first

Sources

“A major triad consists of a major plus a minor third. In the minor chord, this reverses. Note that a major third spans a gap of four semitones, and a minor third spans a gap of three semitones.”

corpus · michael-hewitt-music-theory-for-computer-musicians · chunk 19

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