The atrium of the common Roman townhouse was both a reception hall and a living room, and was either roofless or partly roofed over, with an impluvium or pool in the centre. The compluvium or roofless opening, provided the dwelling with both light and air.
This attractive feature of Roman homes likely has its roots in Roman's own ancestors, the Etruscans. Evidence found in Etruscan tombs indicates that the Etruscans also built their dwellings around a central courtyard or hall. An old Etruscan box of ashes discovered at Poggio Gjello was by design, meant to represent a house, with protruding roof, doors and an impluvium. This architectural feature appears to be completely Italian in origin, differing completely with Greek homes.
Branching off from the atrium one could find the tablinium or office, containing the tabulae, or family records, and the imagines, or ancestor portraits. Some cubiculae or bedrooms might also open onto the atrium, also triclinia (dining rooms), diatae outdoor 'rooms' meant for relaxation, oeci or reception rooms, a kitchen and a lavatory. More lavish homes might also have such luxuries as bath suites and libraries.
References: The Romans: their Life and Customs, E. Guhl and W. Korner (Senate Press, 1994); Handbook to Life in Ancient Rome, Lesley Adkins and Roy A. Adkins (Oxford University Press, 1998)