re avenue and two side aisles, and these usually are
of the same width and height as the nave and its aisles. Sometimes
there are no transepts; sometimes they do not project beyond the line
of the walls, but still are marked by their rising above the lower
height of the nave aisles. Sometimes the transepts have no aisles, or
an aisle only on one side.[1] On the other hand, it is sometimes
customary, especially in English examples, to form two pairs of
transepts. This occurs in Lichfield Cathedral.

[Illustration: FIG. 4.--CHOIR OF WORCESTER CATHEDRAL. (BEGUN 1224.)
A. Nave Arcade. B. Triforium. C. Clerestory.]

[Illustration: FIG. 5.--NAVE OF WELLS CATHEDRAL. (1206 TO 1242.)
A. Nave Arcade. B. Triforium. C. Clerestory.]

The eastern arm of the cathedral is the part to which most importance
was attached, and it is usual to mark that importance by greater
richness, and by a difference in the height of its roof or vault as
compared with the nave; its floor is always raised. It also has its
central passage and its aisles; and it has double aisles much more
frequently than the nave. The eastern termination of the cathedral is
sometimes semicircular, sometimes polygonal, and when it takes this
form it is called an apse or an apsidal east end; sometimes it is
square, the apse being most in use on the Continent, and the square
east end in England. Attached to some of the side walls of the church
it is usual to have a series of chapels; these are ordinarily chambers
partly shut off from the main structure, but opening into it by arched
openings; each chapel contains an altar. The finest chapel is usually
one placed on the axis of the cathedral, and east of the east end of
the main building; this is called, where it exists, the Lady Chapel,
and was customarily dedicated to the Virgin. Henry VII.'s Chapel at
Westminster (Fig. 6) furnishes a familiar instance of the lady chapel
of a great church. Next in importance rank the side chapels which open
out of the aisles of the apse, when there is one. Westminster Abbey
furnishes good examples of these also. The eastern wall of the
transept is a favourite position for chapels. They are less frequently
added to the nave aisles.

The floor of the eastern arm of the cathedral, as has been pointed
out, is always raised, so as to be approached by steps; it is inclosed
by screen work which shuts off the choir, or inclosure for the
performance of divine service, from the nav