actually, because of how lines work, a pentagram has 5 corners.
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Actually, because the shape has not only five corners, but five vertices of the inner pentagon as well, a pentagram has 10 corners. It is like five triangles placed around a 5-cornered pentagon. A pentagram keeps the 5 corners from the pentagon and adds each of the points of each triangle which are also corners, albeit much smaller ones than in the pentagon.