These geometric systems explain the proportional relationships
which are found in the overall shape of the house site and its
organization and subdivision. The dame geometry and dimensions
based on the "regulating square" of the house as a whole appears
to determine the proportions of volumes of space throughout the
house. These systems apply to the composition of the wall
painting and pavement patterns within the rooms as well.
The house was of great importance to the Romans. The domus was
one of their earliest building types and remained the archetype
for other buildings. As such, it embodies the basic
characteristics of Roman design, including an emphasis on a
center. Both ad quadratum and the sacred cut are systems which
emphasize a center through concentric reiterations of the same
geometric operation. The emphasis on center is important in the
domus at a variety of scales, for example in the central atrium
space, centered wall painting compositions, or central floor
mosaics. The Roman world view, placing themselves in the center
of the universe, permeates their design at all scales, including
the organization of the empire, the layout of cities, and the
design of most building types As a basis for the conception and
construction of the domus, the square thus serve a symbolic as
well as pragmatic role.
The domus is the ancient Roman single-family urban house type,
best known from the excavations at Pompeii and Herculaneum.
Analysis of a number of houses at these sites suggests that two
simple geometric systems, both based on the square underlie the
design of the Roman house at all scales. One of these is the ad
quadratum, or square root of 2 progression. A related system is
known as the "sacred cut." The only surviving written treatise
by a Roman architect, that of Vitruvius, does not specifically
discuss the geometric systems to which I refer, but does mention
several times the importance of geometry to the architect.

