Diminished and augmented triads
Build the two tense triads, name their intervals and recognise how they sound.
The diminished triad
Stack two minor thirds and you get a "diminished triad". Its outer notes form not a perfect fifth but a diminished one, a semitone smaller (6 semitones). It sounds tight and uneasy, wanting to resolve.
There is one diminished triad on the white keys, on B: B, D, F.
A diminished triad on B3: two minor thirds
B3, D4, F4 on the keyboard
Practice: Which triad is it?
The augmented triad
Two major thirds make an "augmented triad". Its outer notes form an augmented fifth, a semitone larger than perfect (8 semitones). It sounds unstable, as if hanging in the air.
There is no augmented triad on the white keys: it always needs an accidental. On C it is C, E, G♯.
An augmented triad on C4: two major thirds
So there are four kinds of triad:
Triad Thirds, bottom up Fifth
major triad major + minor perfect minor triad minor + major perfect diminished triad minor + minor diminished augmented triad major + major augmented
The augmented triad splits the octave into three equal parts of 4 semitones. That is why on C, E and G♯ it uses the very same keys.