English: A simplicial arrangement of 20 lines formed by the sides and symmetry axes of a regular decagon. An arrangement is simplicial when, taken as a subdivision of the projective plane, every region has three sides. The same construction produces a simplicial arrangement for any regular polygon. For polygons with even numbers of sides, such as this one, the line at infinity of the projective plane can be added to the arrangement to produce another simplicial arrangement, in this example an arrangement of 21 lines.
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Captions
A simplicial arrangement of lines formed by the sides and symmetry axes of a regular decagon