In block stone arches (see Fig. 3, p. 4) the voussoirs are always cut to a wedge shape.
The curve of the arch having been set out full size on a board, and the number of stones and thickness of arch having been decided, the intrados is divided into as many parts as there are stones, and lines drawn from the centre through these points, till they cut the extrados, give the sides of the voussoirs.
In setting stone arches the space to be occupied by each voussoir - not forgetting the thickness of the joints - is carefully laid out on the centre,1 and the position of the stones checked as they are set.
The stones should be set alternately on each side of the centre, so as to weight it evenly.
The keystone should be carefully fitted at the last before it is set, and driven gently into its place with a few taps of a mallet.
When the arch is so long in plan that one stone cannot extend through from front to back, the work must be built with a regular bond along the soffit. The voussoirs are kept at the same width all through, but of different lengths, so as to break the bond in the length of the arch.
Rubble Arches are built of smaller stones, generally roughly dressed to the wedge shape.