er one, or a window, and the masonry type of the Venetian
Gothic window is consequently three-pieced, _c2_.

Sec. XVI. The reader knows already where a cusp is useful. It is wanted,
he will remember, to give weight to those side stones, and draw them
inwards against the thrust of the top stone. Take one of the side stones
of _c2_ out for a moment, as at _d_. Now the _proper_ place of the cusp
upon it varies with the weight which it bears or requires; but in
practice this nicety is rarely observed; the place of the cusp is almost
always determined by aesthetic considerations, and it is evident that the
variations in its place may be infinite. Consider the cusp as a wave
passing up the side stone from its bottom to its top; then you will have
the succession of forms from _e_ to _g_ (Plate III.), with infinite
degrees of transition from each to each; but of which you may take _e_,
_f_, and _g_, as representing three great families of cusped arches. Use
_e_ for your side stones, and you have an arch as that at _h_ below,
which may be called a down-cusped arch. Use _f_ for the side stone, and
you have _i_, which may be called a mid-cusped arch. Use _g_, and you
have _k_, an up-cusped arch.

Sec. XVII. The reader will observe that I call the arch mid-cusped, not
when the cusped point is in the middle of the curve of the arch, but
when it is in the middle of the _side piece_, and also that where the
side pieces join the keystone there will be a change, perhaps somewhat
abrupt, in the curvature.

I have preferred to call the arch mid-cusped with respect to its side
piece than with respect to its own curve, because the most beautiful
Gothic arches in the world, those of the Lombard Gothic, have, in all
the instances I have examined, a form more or less approximating to this
mid-cusped one at _i_ (Plate III.), but having the curvature of the cusp
carried up into the keystone, as we shall see presently: where, however,
the arch is built of many voussoirs, a mid-cusped arch will mean one
which has the point of the cusp midway between its own base and apex.

The Gothic arch of Venice is almost invariably up-cusped, as at _k_.
The reader may note that, in both down-cusped and up-cusped arches, the
piece of stone, added to form the cusp, is of the shape of a scymitar,
held down in the one case and up in the other.

Sec. XVIII. Now, in the arches _h_, _i_, _k_, a slight modification has
been made in the form of the central piece, i