Stack thirds on a root and you get a chord; its quality (major, minor,
diminished, augmented, and the seventh-chord family) is set by which thirds. Turn it on its side —
put a different chord tone in the bass — and it's an inversion, named by the
figured-bass numbers below. Build one from a root and quality, or type a jazz symbol like
Cmaj7, G7♭9, or Dm7♭5.