The triclinium, or dining room, took its name from the three couches on which family members and their guests lounged to take their meals.
Each couch was wide enough to accommodate three diners who reclined on their left side on cushions while some household slaves served multiple courses rushed out of the culina, or kitchen and others entertained guests with music, song or dance.
Marcus Vitruvius Pollio, in his book, de Architectura, describes the triclinium: "The length of a triclinium is to be double its breadth. The height of all oblong rooms is thus regulated: add their length and breadth together, of which take one half, and it will give the dimension of the height. If, however, exedræ or oeci are square, their height is equal to once and a half their width. Pinacothecæ (picture rooms), as well as exedræ, should be of large dimensions. The Corinthian tetrastyle and Egyptian oeci (halls) are to be proportioned similarly to the triclinia, as above described; but inasmuch as columns are used in them, they are built of larger dimensions."
He goes on to direct the orientation of triclinia according to the seasons: "Winter triclinia and baths are to face the winter west, because the afternoon light is wanted in them; and not less so because the setting sun casts its rays upon them, and but its heat warms the aspect towards the evening hours....Spring and autumn triclinia should be towards the east, for then, if the windows be closed till the sun has passed the meridian, they are cool at the time they are wanted for use. Summer triclinia should be towards the north, because that aspect, unlike others, is not heated during the summer solstice, but, on account of being turned away from the course of the sun, is always cool, and affords health and refreshment."
The social status of diners would immediately be apparent upon entering the triclinium. Each of the three couches in the triclinium carried significant social standing, as did the three positions on the couch. The lectus imus was the master's couch, and he assumed the locus summus. To his immediate right was the locus medius, and farthest from him on the couch was the locus imus. Members of the host's own family, if they dined with him, usually reclined with him on this couch, which held the least status.
Guests were positioned on the other two couches at the master's discretion and according to their social standing. To his immediate left was the lectus medius, and the third position upon that couch, the locus imus, was the place of honour, or locus consularis. The guest placed here had the greatest access to the host.
References: Handbook to Life in Ancient Rome, Lesley Adkins and Roy A. Adkins (Oxford University Press, 1998); The Romans, their Life and Customs, E. Guhl and W. Korner, Senate Press, 1994