The symbol above shows a sixfold inversion axis, i.e. a sixfold
inversion axis perpendicular to the screen.
A fractional value next to the symbol indicates the height of the inversion
centre above the XY plane.
As with the sixfold axis, a positive rotation is anticlockwise.
A sixfold 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 -x+y,-x,-z.
This axis has the written symbol "-6"
(where the minus sign is usually written above the digit).
The solid triangle at the centre of the symbol emphasises the implied
threefold axis that results from a sixfold inversion axis.
