Definition 1

A solid is that which has length, breadth, and depth.

Definition 2

A face of a solid is a surface.


The 28 definitions at the beginning of Book XI serve Books XII and XIII as well.

The first two definitions correspond to definitions I.Def.2 and I.Def.3 for a line and its ends, and definitions I.Def.5 and I.Def.6 for a surface and its edges.

Some examples of solids that appear in Books XI through XII are: