Chord qualities
A chord quality is what you hear when three or more notes sound together: major, minor, diminished, and everything built on top of them. The root can change without the quality changing — it is the shape, not the pitch, that gives a chord its colour.
The twelve below cover what turns up in most music. They are ordered from the triads, which have three notes, to the four-note chords built by stacking one more third.
Major M C · E · G 1 M3 P5 Minor m C · D♯ · G 1 m3 P5 Diminished ° C · D♯ · F♯ 1 m3 ♭5 Augmented + C · E · G♯ 1 M3 ♯5 Dominant seventh 7 C · E · G · A♯ 1 M3 P5 m7 Major seventh ∆ C · E · G · B 1 M3 P5 M7 Minor seventh m7 C · D♯ · G · A♯ 1 m3 P5 m7 Half-diminished ø C · D♯ · F♯ · A♯ 1 m3 ♭5 m7 Diminished seventh °7 C · D♯ · F♯ · A 1 m3 ♭5 dim7 Major sixth 6 C · E · G · A 1 M3 P5 M6 Suspended fourth sus4 C · F · G 1 P4 P5 Dominant ninth 9 C · E · G · A♯ · D 1 M3 P5 m7 M9