The Vitruvian doorway
Vitruvius IV.6: the height of a temple door from the height of the room, its width from its height, and a taper toward the top.
The rules it illustrates
The door from the height of the room. Divide the height from floor to ceiling into three and a half parts and give two and a half to the opening; make a Doric door five and a half twelfths as wide as it is high (an Ionic one two-fifths), and narrow it slightly at the top.
Ears on a lintel over sloping jambs. When a window or door narrows towards the top and has no cornice, let the ends of the lintel run out beyond the jambs as ‘ears’ (crossettes), so that the head is firmly marked.
Where it appears in the book
In the chapters Walls and Openings.