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A face's
sidedness indicates whether it is single-sided or double-sided. A
single-sided face means that points on one side of it are considered to be inside the shell, and points on the other side are considered to be on the outside. The selection of the inside and outside may be the same as, or converse to, that for the surface of which the face is part--depending on the sense of the face. A single-sided face either completely or partially bounds a solid region, with the face normal pointing away from the solid region. A
double-sided face means that the points on either side are either
all inside or
all outside. If they are all outside, the face is considered to represent an idealized, infinitely thin sheet (a 2D region). If they are inside, the face is an internal partition embedded in a solid. (A single-sided face is a solid body, not a sheet body.)
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