3/ Now look at what they've done for "hedra". Base, chair, seat, face of a geometric solid. One of these is not like the others.    "Hedra" is not a word we use for chair. We use it for shapes — polyhedron.    So "hedra" here doesn't mean "chair" — it means "box" or maybe even "plane" in the sense of a single plane of something that has multiple sides/planes.