Diminished 7th chords
Stack three minor thirds in a row and the seventh shrinks all the way to a double-flatted seventh, which lands on the same key as a plain sixth. The chord is perfectly symmetrical, so the same four keys can be renamed and reused as a diminished seventh built on any of its four notes. It's the classic villain-sting and passing chord of common-practice harmony.
Other chord types
MajorMinorDiminishedAugmentedSuspended 2ndSuspended 4thDominant 7thMajor 7thMinor 7thMajor 6thMinor 6thMinor Major 7thHalf-Diminished 7thAugmented 7thAdd9Dominant 9thMajor 9thMinor 9th6/9Dominant 11thMinor 11thDominant 13thMajor 13thMinor 13th7sus47b57b97#9Dominant 7#11Minor 6/99sus4Power Chord (5)Minor Add9Add117sus213sus4Minor Major 9thMajor 7#11Major 9#117b1313b913#117altAugmented Major 7thMinor 9b5Quartal (So What)