Diminished and Augmented Chords: The Other Two Triads

The short answer: There are four triad types, not two. Diminished = a minor chord with the fifth lowered a semitone (1–♭3–♭5). Augmented = a major chord with the fifth raised a semitone (1–3–♯5). Squeezing or stretching the fifth removes the triad's stable frame, so both sound tense and unresolved — which is exactly what they're for: they're the chords that make the next chord feel inevitable.

Built from stacked thirds

Every triad is two thirds stacked up, and the four types are just the four ways to stack major and minor thirds:

Triad	Stack	Spelling from C/B	Sound
Major	maj 3rd + min 3rd	C–E–G	home
Minor	min 3rd + maj 3rd	C–E♭–G	sad home
Diminished	min 3rd + min 3rd	B–D–F	anxious, leaning
Augmented	maj 3rd + maj 3rd	C–E–G♯	dreamlike, floating

Here's B diminished as a compact shape on the top three strings, next to C augmented one fret cluster over:

B diminished (B–D–F): two minor thirds stacked

C augmented (C–E–G#): two major thirds stacked

You already play a diminished chord (sort of)

Harmonize any major scale and the chord built on the 7th degree comes out diminished — in C major, that's B°. It's the answer to "why is the 7 chord weird", and it's why the diminished chord always sounds like it's about to be the I chord: B° is basically a G7 missing its root, and both ache to resolve to C. In real songs, diminished chords mostly work as passing chords between diatonic neighbors — the classic move is I → ♯i° → ii (C → C♯° → Dm), a chromatic stepping stone you'll hear all over jazz standards, gospel, and old pop.

The symmetry party trick

Diminished and augmented triads divide the octave into equal slices — minor thirds (3 frets) and major thirds (4 frets) respectively. Consequence on the fretboard: move a diminished 7th chord shape up 3 frets and you get the same chord, respelled. Move an augmented shape up 4 frets, same deal. No other chords do this. It's why diminished runs sound like a staircase with no floor ("spooky silent-movie chord") and why the same aug shape covers three different chord names — the fretboard economy is absurd.

Augmented in the wild

Rarer than diminished, but unmistakable: the opening chord stab of The Beatles' "Oh! Darling," the James Bond theme's final chord, and the classic I → I+ → vi move ("Baby Hold On") where the fifth creeps up a semitone as a melodic elevator. Augmented chords almost always appear this way — one voice sliding chromatically while the rest hold — rather than as destinations.