This article discusses cathedral diagrams:
For comparison, the follwoing is the plan of Tewkesbury Abbey with the corresponding parts highlighted in the same colors. (Note: They are not drawn to the same scale; they are drawn to be about the same length in the diagram.)
In these two cruciform (cross shaped) buildings the arms of the cross (together, the "transept," which formed an aisle across the building) are quite pronounced; however, the transept arms might be so short as not to stick out past the sides of the building (as at Notre-Dame de Paris), or there might be two of them (as at Canterbury Cathedral and Salisbury Cathedral).
Some gothic churches - such as Bourges - had no transepts at all and thus were not cruciform.
The end with the altar in it was normally at the east, for symbolic religious reasons, though frequently the building could not be disposed in such a way as to make that orientation very precise. The main doors were at the west end, and there were often towers on that end and usually an opening into the nave - often a stained glass "rose window."
At the ends of the transept were doors, too, and outside them were porches that were used for various rituals;
The semi-circular end of the church around the high altar, which corresponds to the apse in Romanesque and earlier architecture, was often expanded into a passage called an ambulatory (from the Latin to walk) and chapels disposed around the ambulatory.
Users could make a complete circuit within the building using the north and south aisles of the nave and the ambulatory. In the bays around the ambulatory, between the supporting columns, were shrines and chapels. There was usually a larger chapel on the east end of the axis of the church - often dedicated to the patron saint of the church, or to Mary, the mother of Jesus - at the far east end. In medieval English usage this was called a Lady Chapel.
"Chantries" were shrines or chapels where someone had paid an "endowment" to have the monks say (or "chant") prayers on a fixed schedule for someone who had died.
The "nave" (from the Latin for a ship) was the part directly inside the main (west) doors where the public would attend services. There was usually a "rood screen" ("rood" = "cross" or "crucifix") across the nave, partitioning off the "choir" (earlier, "quire"), which was where monks would attend their own services (or "offices"). Against the screen, on its west side toward the nave where the public could see it, was usually an altar. The next section to the east after the choir was the "presbytery" (meaning "priestly") where the priests who were assisting at Mass would sit; that section was not usually separate and might be only a couple of fancy chairs at the side. The heart of the building was the "sanctuary" where the "high altar" was. There would be altars in many of the chapels, but this was the one where Mass would be said for the public. This area was also where criminals seeking the right of sanctuary were safe from the law.
This picture shows the inside of Salisbury Cathedral, looking west from behind the high altar. The sanctuary, presbytery, choir, and nave are visible, with the back of the west front in the background.
The apse did not last long as an architectural fashion; in Europe it was replaced by the rounded "chevet," and in England by squared-off east ends, and as the cathedrals were rebuilt or repaired, their apses were remodeled into the newer shapes.
Outside the cathedral would be the "chapter house" where the monks or priests whose church it was would hold their meetings about church business; chapter houses were often round and were always connected to the church building. There was also usually a "cloister," a rectangular colonnade around a grass lawn, where the monks could walk, and their work or study cubicles often opened onto it.
See also: Cathedral architecture