Extinct: The third caster.
"Traditional cruet stands came with three matching casters—
that is, bottles with perforated tops for sprinkling (or casting)
flavourings onto food. Two of the casters contained salt and
pepper, but what went into the third caster is unknown. It is
generally presumed to be dried mustard, but that is really
because no one can think of anything more likely."
—Bill Bryson, At Home