The first level of the bay is where the piers or columns separate the nave from the aisles. The aisles are between the nave and the outside walls and there is a north aisle and a south aisle. Aisles provide space for additional worshipers, for side chapels, and for liturgical processions.
From the outside, we can see that the aisles have lower vaults than the nave, also known as the central vessel.
A series of bays create what we call an arcade.
On the left you can see the row of bays creating an arcade. A related, but different architectural feature is called a blind arcade, or a blank arcade. This occurs with you have a series of decorative arches (usually interlacing arches) on a wall. There is no aisle behind these arches.
What we call the second level depends on its usefulness. In older Romanesque churches (call Norman in England), the second level is called a gallery because it offers usable space for side chapels, processions, or seating worshipers.
Later, liturgical customs changed and all worship activities took place on the first floor. Therefore, the second level lost its usefulness about the same time Gothic architecture became the new style, so there was no longer a need to have a functional second level. In most Gothic churches, this second floor will look like your unfinished attic. Viewed from the nave, we can see a tier of smaller arches, but these provide only the illusion of a useful second level. In Gothic churches, therefore, we call this level a triforium.
Both galleries and the triforium “attic” space are above the aisle vaults.
The third, or top level, is called the clerestory. Its function, besides providing architectural loveliness, is to provide light to come into the structure.
Let’s return to our floor plan. The north and south extensions from the building are called transepts. English transepts are characteristically longer than those found on the continent.
In medieval times, the north transept tended to be the entrance for pilgrims and the south tended to be entrance for monks and clergy.
Besides giving the building a cruciform shape, transepts help support the central crossing tower as well as providing more space for side chapels.
You can again get a sense for the size of these buildings by looking at the size of the people in this photo.
Some cathedrals have two sets of transepts, more common in England. You might also note that the east end is flat, rather than the apse shape we have been looking at in our generic floor plan.
The crossing is the area of a cathedral where the east and west sections intersect with the north and south transepts. Usually, a crossing tower rises above, providing more light to enter the church.