Diminished triads (3+3 semitones) and augmented triads (4+4 semitones) are the two unstable triad types outside major and minor

All four triad types arise from stacking two thirds. Major = major third + minor third (4+3 semitones). Minor = minor third + major third (3+4 semitones). Diminished = two minor thirds (3+3 semitones) — contains a diminished fifth/tritone, producing an unstable, eerie sound. Augmented = two major thirds (4+4 semitones) — contains an augmented fifth, equally unstable. Diminished triads appear as chord vii in major keys. Augmented triads appear on chord III in harmonic minor.

Examples

B diminished: B D F (3+3 semitones). C augmented: C E G# (4+4 semitones). Notation: Bdim; Caug.

Assessment

Build F# diminished and Ab augmented from scratch using the semitone rule. Explain what makes each unstable.