Centre of Inversion on Fourfold
Axis (4 / m)
The symbol above shows a fourfold axis
perpendicular to the plane of the screen, i.e parallel to Z, with a centre
of inversion on the axis.
This combination results when there is a mirror plane
perpendicular to the fourfold axis,
hence the written symbol "4 / m".
A fractional value next to the symbol indicates the height of the inversion
centre above the XY plane.
Although not shown, a fourfold inversion axis
(-4) is also present with its inversion point at the
same height as the centre of inversion.