The symbol above shows a threefold inversion axis, i.e. a threefold axis
perpendicular to the screen with a centre of inversion on it.
A fractional value next to the symbol indicates the height of the inversion
centre above the XY plane.
As with the threefold axis, a positive rotation is anticlockwise.
A threefold rotation about the c-axis, i.e. about
the line 0,0,z, followed by an inversion at the origin will have
the corresponding symmetry operator y,-x+y,-z.
This axis has the written symbol "-3"
(where the minus sign is usually written above the digit).