Centre of Inversion on Twofold Screw Axis (21 / m)

2(1)/m

The symbol above shows a two-one screw 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 two-one screw axis, hence the written symbol "21 / m".

A fractional value next to the symbol indicates the height of the inversion centre above the XY plane. The mirror plane is separated from the centre of inversion by 1/4 of the repeat distance in the lattice along the axis direction.

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© Copyright 1995-2006.  Birkbeck College, University of London. Author(s): Jeremy Karl Cockcroft