The general arrangements of a Roman house were as follows: next the street an open space was frequently left, with porticoes on each side of it provided with seats: this constituted the vestibule, and was entirely outside the house;[22] the entrance-door opened into a narrow passage, called the _prothyrum_, which led to the _atrium_,[23] which in the houses of Republican Rome was the principal apartment, though afterwards it served as a sort of waiting-room for the clients and retainers of the house; it was an open court, roofed in on all the four sides, but open to the sky in the centre. The simplest form was called the Tuscan atrium, where the roof was simply a lean-to sloping towards the centre, the rafters being supported on beams, two of which rested on the walls of the atrium, and had two other cross-beams trimmed into them. The centre opening was called the _impluvium_, and immediately under it a tank, called the _compluvium_, was formed in the pavement to collect the rain-water (Fig. 142). When the atrium became larger, and the roof had to be supported by columns, it was called a _cavædium_.[24] At the end of this apartment were three others, open in front, the largest, in the centre, called _tablinum_, and the two side ones _alæ_;[25] these were muniment-rooms, where all the family archives were kept, and their position is midway between the semi-public part of the house, which lay towards the front, and the strictly domestic and private part, which lay in the rear. At the sides of the atrium in the larger houses were placed small rooms, which served as sleeping chambers. [Illustration: FIG. 142.--THE ATRIUM OF A POMPEIAN HOUSE.]