ing the distances apart of the piers of the main
arcade that the compartments (usually termed bays) of the aisle should
be square on plan; and it was quite possible, without doing more than
the Romans had done, to vault each bay of the aisles with a
semicircular intersecting vault (_i.e._ one which has the appearance
of a semicircular or waggon-head vault, intersected by another vault
of the same outline and height). This produced a simple series of what
are called groined or cross vaults, which allowed height to be given
to the window heads of the aisle and to the arcades between the aisles
and nave.

After a time it was desired to vault the nave also, and to adopt for
it an intersecting vault, so that the heads of the windows of the
clerestory might be raised above the springing line of the vault, but
so long as the arches remained semicircular, this was very difficult
to accomplish.

The Romans would probably have contented themselves with employing a
barrel vault and piercing it to the extent required by short lateral
vaults, but the result would have been an irregular, weak, curved line
at each intersection with the main vault; and the aisle vaults having
made the pleasing effect of a perfectly regular intersection familiar,
this expedient does not seem to have found favour, at any rate in
England.

Other expedients were however tried, and with curious results. It was
for example attempted to vault the nave with a cross vault, embracing
two bays of the arcade to one of the vault, but the wall space so
gained was particularly ill suited to the clerestory windows, as may
be seen by examining the nave of St. Stephen's at Caen. In short, if
the vaulting compartment were as wide as the nave one way, but only as
wide as the aisle the other way, and semicircular arches alone were
employed, a satisfactory result seemed to be unattainable.

In the search for some means of so vaulting a bay of oblong plan that
the arches should spring all at one level, and the groins or lines of
intersection should cross one another in the centre of the ceiling,
the idea either arose or was suggested that the curve of the smaller
span should be a pointed instead of a semicircular arch.

The moment this was tried all difficulty vanished, and groined (_i.e._
intersecting) vaults, covering compartments of any proportions became
easy to design and simple to construct, for if the vault which spanned
the narrow way of the compartment were ac