Wherever arches are used in groups care must be exercised in laying out the springing stones to give a level support for the spandrels. Thus where two arches come together, as at A, Fig. 105, if the first voussoir is cut to the shape of the arch on the back a small wedge-shaped piece of stone would be required to fill the space between the first pair of voussoirs. The weight of the wall above coming on this wedge might be sufficient to force the voussoirs in and seriously mar the appearance of the arch, as well as causing cracks in the ashlar above. This danger may be overcome by cutting the lower stone, a a, in one piece for both arches and extending the voussoir, b, to a vertical joint over the centre of the pier. This gives a level bearing for the lower stone in the spandrel and effectually prevents any pushing in of the voussoirs.

Fig. 105.

Another case very similar to this often occurs where the back of an arch comes almost to the corner of the wall or projection, as shown at B. If the distance between the back of the arch and the angle of the wall is less than 8 inches the lower voussoir should be cut the full width of the pier, as shown in the illustration.