The following illustration of a generic box appears on p. 63 of the TeXbook:
What is a box's reference point in the following pathological cases?
The box has either (a) no width, or (b) no height and no depth, or (c) both a and b?
The box has a positive width and either a positive height or a positive depth (or both), which were not explicitly assigned but calculated automatically, and one of the following cases holds.
- The box doesn't contain sub-boxes (but may contain other elements, e.g. glue)?
- The box contains at least one sub-box, but some of the sub-boxes -- possibly all of them! -- is shifted to the left or to the right, or raised or lowered?