A basilica was a spacious hall which served as an extension of a market place, and was itself in a certain sense a covered market. It was not limited to a specific purpose; in general, whatever took place on the market square might take place in the basilica, the roof of which afforded protection against the weather. It was chiefly devoted, however, to business transactions and to the administration of justice. The form is known partly from the remains of the basilicas in Rome--Basilica Julia, Basilica Ulpia, the Basilica of Constantine--and in Africa, but more fully from the treatise of Vitruvius and the description of a basilica which he himself erected at Fano. [Illustration: Fig. 23.--Plan of the Basilica. _a._ Entrance court. 1. Corridor. 2. Main room. 3. Tribunal. 4. Rooms at the ends of the tribunal.] According to these sources the plan of a typical basilica is essentially that of the building before us (Fig. 23). An oblong space is divided by columns into a broad central hall and a corridor which runs around the four sides. The height of the columns, in the typical basilica, is equal to the width of the corridor, which is covered by a flat roof; the inner edge of this roof is carried by the entablature above the columns. The main room is higher than the corridor. Above the entablature is a low wall on which there is a second row of columns; these carry the main roof and form a clerestory, the light being admitted through the intercolumniations. The main hall and the corridor were devoted to trade; the dealers perhaps occupied the former, while in the latter the throng of purchasers and idlers moved freely about. The place set aside for the administration of justice, the tribunal, was ordinarily an apse projecting from the rear end. In our Basilica, however,--and in some others as well,--it was a small oblong elevated room back of the central hall, toward which it opened in its whole length. This ideal plan would answer very well for that of the early Christian basilicas, excepting in one respect; instead of a corridor on all four sides they have only aisles parallel with the nave, an arrangement which had already been adopted in some basilicas designed for markets. The Christian basilicas would give us a still truer idea of the arrangement and lighting of the pagan prototype if in most cases a part of the numerous windows had not been walled up, thus producing a dimness in keeping with a religious but not a secular edifice. In pagan structures the ideal plan was by no means strictly followed. Vitruvius himself at Fano, and the architects of other basilicas the remains of which have been discovered, did not hesitate to depart from it. So the Basilica at Pompeii, as we shall see, presents a modification of the general plan in an important particular, the admission of light; and this deviation was carried out with finer artistic feeling than was displayed by Vitruvius in his building.