The large public room at the center of the domus is the atrium. Other rooms of the domus typically adjoin or open into it. It contains a roofless space called the compluvium which provides air and light. This opening also allows water that collects on the roof to drain into a basin positioned below it called the impluvium.

In the early Italian houses, which had a covered atrium, the alae (singular = ala, "wing") were the open rooms on each side of the atrium, and they had windows to allow light to enter the house. However, with the introduction of the compluvium and the general abandoning of windows in the Roman house, the alae became largely obsolete. It appears that they were incorporated into the house more in accordance to tradition than for any specific use.