1. For the doorways of temples and their casings the rules are as follows, first determining of what style they are to be. The styles of portals are Doric, Ionic, and Attic.

In the Doric, the symmetrical proportions are distinguished by the following rules. Let the top of the corona, which is laid above the casing, be on a level with the tops of the capitals of the columns in the pronaos. The aperture of the doorway should be determined by dividing the height of the temple, from floor to coffered ceiling, into three and one half parts and letting two and one half[4] thereof constitute the height of the aperture of the folding doors. Let this in turn be divided into twelve parts, and let five and a half of these form the width of the bottom of the aperture. At the top, this width should be diminished, if the aperture is sixteen feet in height, by one third the width of the door-jamb; if the aperture is from sixteen to twenty-five feet, let the upper part of it be diminished by one quarter of the jamb; if from twenty-five to thirty feet, let the top be diminished by one eighth of the jamb. Other and higher apertures should, as it seems, have their sides perpendicular.

2. Further, the jambs themselves should be diminished at the top by one fourteenth of their width. The height of the lintel should be equivalent to the width of the jambs at the top. Its cymatium ought to be one sixth of the jamb, with a projection equivalent to its height. The style of carving of the cymatium with its astragal should be the Lesbian. Above the cymatium of the lintel, place the frieze of the doorway, of the same height as the lintel, and having a Doric cymatium and Lesbian astragal carved upon it. Let the corona and its cymatium at the top of all be carved without ornamentation, and have a projection equal to its height. To the right and left of the lintel, which rests upon the jambs, there are to be projections fashioned like projecting bases and jointed to a nicety with the cymatium itself.

3. If the doorways are to be of the Ionic style, the height of the aperture should be reached in the same manner as in the Doric. Let its width be determined by dividing the height into two and one half parts and letting one of them form the width at the bottom. The diminutions should be the same as for Doric. The width of the faces of the jambs should be one fourteenth of the height of the aperture, and the cymatium one sixth of the width. Let the rest, excluding the cymatium, be divided into twelve parts. Let three of these compose the first fascia with its astragal, four the second, and five the third, the fasciae with their astragals running side by side all round.

4. The cornices of Ionic doorways should be constructed in the same manner as those of Doric, in due proportions. The consoles, otherwise called brackets, carved at the right and left, should hang down to the level of the bottom of the lintel, exclusive of the leaf. Their width on the face should be two thirds of the width of the jamb, but at the bottom one fourth slenderer than above.

Doors should be constructed with the hinge-stiles one twelfth of the width of the whole aperture. The panels between two stiles should each occupy three of the twelve parts.

5. The rails will be apportioned thus: divide the height into five parts, of which assign two to the upper portion and three to the lower; above the centre place the middle rails; insert the others at the top and at the bottom. Let the height of a rail be one third of the breadth of a panel, and its cymatium one sixth of the rail. The width of the meeting-stiles should be one half the rail, and the cover-joint two thirds of the rail. The stiles toward the side of the jambs should be one half the rail. If the doors have folds in them, the height will remain as before, but the width should be double that of a single door; if the door is to have four folds, its height should be increased.

Vitruvius' Rule For Doorways Compared With Two Examples
vitruvius' rule for doorways compared with two examples

6. Attic doorways are built with the same proportions as Doric. Besides, there are fasciae running all round under the cymatia on the jambs, and apportioned so as to be equal to three sevenths of a jamb, excluding the cymatium. The doors are without lattice-work, are not double but have folds in them, and open outward.

The laws which should govern the design of temples built in the Doric, Ionic, and Corinthian styles, have now, so far as I could arrive at them, been set forth according to what may be called the accepted methods. I shall next speak of the arrangements in the Tuscan style, showing how they should be treated.