hree classes.
The first is laid out by dividing the length into five parts and giving
three parts to the width; the second, by dividing it into three parts
and assigning two parts to the width; the third, by using the width to
describe a square figure with equal sides, drawing a diagonal line in
this square, and giving the atrium the length of this diagonal line.

4. Their height up to the girders should be one fourth less than their
width, the rest being the proportion assigned to the ceiling and the
roof above the girders.

The alae, to the right and left, should have a width equal to one third
of the length of the atrium, when that is from thirty to forty feet
long. From forty to fifty feet, divide the length by three and one
half, and give the alae the result. When it is from fifty to sixty feet
in length, devote one fourth of the length to the alae. From sixty to
eighty feet, divide the length by four and one half and let the result
be the width of the alae. From eighty feet to one hundred feet, the
length divided into five parts will produce the right width for the
alae. Their lintel beams should be placed high enough to make the height
of the alae equal to their width.

5. The tablinum should be given two thirds of the width of the atrium
when the latter is twenty feet wide. If it is from thirty to forty feet,
let half the width of the atrium be devoted to the tablinum. When it is
from forty to sixty feet, divide the width into five parts and let two
of these be set apart for the tablinum. In the case of smaller atriums,
the symmetrical proportions cannot be the same as in larger. For if, in
the case of the smaller, we employ the proportion that belong to the
larger, both tablina and alae must be unserviceable, while if, in the
case of the larger, we employ the proportions of the smaller, the rooms
mentioned will be huge monstrosities. Hence, I have thought it best to
describe exactly their respective proportionate sizes, with a view both
to convenience and to beauty.

[Illustration: _From Mau_

PLAN OF A TYPICAL ROMAN HOUSE]

6. The height of the tablinum at the lintel should be one eighth more
than its width. Its ceiling should exceed this height by one third of
the width. The fauces in the case of smaller atriums should be two
thirds, and in the case of larger one half the width of the tablinum.
Let the busts of ancestors with their ornaments be set up at a height
corresponding to the width of the alae.