Classification
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The simple five-pointed star is not a pentagram; it is a concave decagon.
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A pentagram usually has strokes through the center and has 5 points.
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A pentacle is a pentagram inscribed on a pendant or disc, sometimes this is depicted as a glyph with a circle around it (point-down in this image), but this glyph is not in itself a pentacle.
Read more about this topic: Pentagram