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Next: Examples of Ee planes. Up: Symbols for Symmetry Elements and Symmetry Previous: 1. Printed symbols for symmetry elements

2. Graphical symbols for symmetry elements

The existing graphical symbols as defined and used in ITA83 are considered to be adequate by the Ad-hoc Committee with one exception. The situation is very different from that of printed symbols because in drawings the projection direction becomes an added parameter. In the terms `perpendicular projection' and `parallel projection' used below, perpendicular and parallel refer to the orientation of the projection direction with respect to the glide plane.

For an Ee plane the symbol exists for its perpendicular projection. The existing graphical symbols for the parallel projection of an Ee-glide plane, however, suffer from the same ambiguity as the printed symbols. They express the glide vector either parallel or perpendicular to the projection direction, but the choice is arbitrary. Therefore, a new symbol is proposed for the parallel projection of an Ee glide plane, namely

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