I have done a bit of maths in my time and do agree the preamble was a little bit ambiguous to say the least, and possibly downright wrong.

The accepted term to apply to a 2D shape, is that if an image exhibits horizontal symmetry then a “mirror” line (or axis) can be drawn horizontally through the image (e.g. ‘C’ or ‘D’) and likewise vertical symmetry is exhibited when a vertical line is drawn through the image to the same effect (e.g. A or Omega here). Chemists/ Crystallographers use the terms frequently in 3D, referring to molecules, and probably deserve to be the accepted authority.

The preamble stated symmetric horizontally, which is n’t a term of symmetry I’m aware of, but could mean symmetric along the horizon or “vertically symmetric”, to use the accepted mathematical term or be “plane” wrong (excuse the pun). Certainly not as precise as the clueing. ]]>

This was slow going for me until I hit on the way of entering the words. The one problem here is that there does not seem to be general agreement on what is horizontal and what is vertical symmetry. The definition depends on whom you ask. Or is there a supreme authority on this who can be consulted?

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