Stone masonry is classified according to the shape of the stones, and also according to the quality and accuracy of the dressing of the joints so that the joints may be close. The definitions of these various kinds of stonework have already been given in the previous section, and therefore will not be repeated here; but the classification will be repeated in the order of the quality and usual relative cost of the work. The term ashlar refers to the rectangular shape of the stone and the accuracy of dressing the joints, and may be applied to coursed ashlar, range, and even random. The next grade in quality is squared-stone masonry, which likewise refers only to the accuracy in dressing the joints. The variations in the coursing of the stones may be the same as for ashlar. The term rubble is usually applied to stone masonry on which but little work has been done in dressing the stones, although the cleavage planes may be such that very regular stones may be produced with very little work. Rubble masonry usually has joints which are very irregular in thickness. In order to reduce the amount of clear mortar which otherwise might be necessary in places between the stones, small pieces of stone called spalls are placed between the larger stones. Such masonry is evidently largely dependent upon the shearing and tensile strength of the mortar and is therefore comparatively weak. Random rubble (Fig. 31), which has joints that are not in general horizontal or vertical, or even approximately so, must be considered as a weak type of masonry. In fact the real strength of such walls, which are frequently built for architectural effect, depends on the backing to which the facing stones are sometimes secured by cramps.