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, has 5 corner vertices and intersecting edges, while concave decagon, |5/2|, has with 10 edges and 2 sets of 5 vertices. The first are used in definitions of star polyhedra, while the second are used in planar tilings. |- align=center |160px Small stellated dodecahedron |160px Tessellation |} In geometry, a star polygon (not to be confused with a star-shaped polygon) is a concave polygon. Only the regular star polygons have been studied in any depth; star polygons in general appear not to have been formally defined. Branko Grünbaum identified two primary definitions used by Kepler, one being the regular star polygons with intersecting edges that don't generate new vertices, and a second are simple isotoxal concave polygons. The first usage is included in polygrams which includes polygons like the pentagram but also compound figures like the hexagram. ==Etymology== Star polygon names combine a numeral prefix, such as ''penta-'', with the Greek suffix ''-gram'' (in this case generating the word ''pentagram''). The prefix is normally a Greek cardinal, but synonyms using other prefixes exist. For example, a nine-pointed polygon or ''enneagram'' is also known as a ''nonagram'', using the ordinal ''nona'' from Latin. The ''-gram'' suffix derives from ''γραμμή'' (''grammḗ'') meaning a line.〔(γραμμή ), Henry George Liddell, Robert Scott, ''A Greek-English Lexicon'', on Perseus〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「star polygon」の詳細全文を読む スポンサード リンク
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