Hi Frederick,

re:
> So two faces can share an single edge OR two faces have edges that, not sure how to
> put this, exist in the exactly the same space?

Yup, that's correct. There can be one common shared edge between two faces. This is called a "joined edge" and to have a solid you want to have all edges be in this state.

If two faces are touching at an edge but it isn't joined, then there are 2 separate "naked" edges there that are coincident.


> I'm guessing a face must be, for lack of the correct term, "enclosed" by edges?

Yes, a face has "trim boundaries" (or "loops") on it. Each loop consists of one or more edges forming a closed outline.

A face can have one "outer loop" and any number of "inner loops" making holes.


- Michael