as Aust beautifully expresses it;[379] this was the seat of Vesta, and behind it was the penus or store-closet, the seat of the Penates; thus Vesta and the Penates are in the most genuine sense the protecting and nourishing deities of the household.  Here, too, was the Lar of the familia with his little altar, behind the entrance, and here was the lectus genialis,[380] and the Genius of the paterfamilias.  As you looked into the atrium, after passing the vestibulum or space between street and doorway, and the ostium or doorway with its janua, you saw in front of you the impluvium, into which the rainwater fell from the compluvium, i.e. the square opening in the roof with sloping sides; on either side were recesses (alae), which, if the family were noble, contained the images of the ancestors.  Opposite you was another recess, the tablinum, opening probably into a little garden; here in the warm weather the family might take their meals.

This is the atrium of the old Roman house, and to understand that house nothing more is needed.  And indeed architecturally, the atrium never lost its significance as the centre of the house; it is to the house as the choir is to a cathedral.[381] And it is easy to see how naturally it could develop into a much more complicated but convenient dwelling; for example, the alae could be extended to form separate chambers or sleeping-rooms, the tablinum could be made into a permanent dining-room, or such rooms could be opened out on either side of it.  A second story could be added, and in the city, where space was valuable, this was usually the case.  The garden could be converted, after the Greek fashion, and under a Greek name, into a peristylium, i.e. an open court with a pretty colonnade round it, and if there were space enough, you might add at the rear of this again an exedra, or an oecus, i.e. open saloons convenient for many purposes.  Thus the house came to be practically divided into two parts, the atrium with its belongings, i.e. the Roman part, and the peristylium with its developments, forming the Greek part; and the house reflects the composite character of Roman life in its later period, just as do Roman literature and Roman art.  The Roman part was retained for reception rooms, and the Lar, the Penates, and Vesta, with their respective seats, retired into the new apartments for privacy.  When the usual crowd of morning callers came to wait upon a great man, they would not as a rule penetrate farther than the atrium, and there he might keep them waiting as long as he pleased.  The Greek part of the house, the peristylium and its belongings, was reserved for his family and his most intimate friends.  In Pompeii, which was an old Greek town with Roman life and habits superadded, we find atrium and peristylium both together as early as the second century B.C.[382] At what period exactly the house of the noble in Rome began thus to develop is not so certain.  But by the time of Cicero every good domus had without doubt its private apartments at the rear, varying in shape and size according to the ground on which the house stood.[383]