Vertical joints are staggered in various ways (see the chart in this chapter) so that any stress and strain will not be likely to loosen more than one or two courses of masonry units. In many modern designs this construction principle is varied so that the units are "stacked," or laid one above the other, to give strong vertical as well as strong horizontal lines. Usually this is done only on veneered walls or on low walls where little stress is likely to occur. The loss in strength is somewhat compensated for by the insertion of metal tie rods or tie strips in the mortar to span the vertical joints. In ordinary bonds, the staggering is most frequently to the exact center of the unit in the course below and above, but sometimes for effecting various patterns the staggering is varied to one-third or one-quarter of the distance across the unit below.

Ancient bricks were hand-made and varied widely in size and even in shape. They were rough and probably rather difficult to lay, yet we find them in walls which are still standing after many centuries, so we can see how durable masonry can be. In Pompeii, in Rome, and in other cities of even greater antiquity, we see many walls of great beauty made with sun-baked brick. With today's machine-made bricks and other precisely formed masonry units which are almost identical in shape and size, varying only a tiny fraction due to shrinkage in baking or drying, we find it quite easy to obtain walls that are pleasantly uniform and which conform to exact measurements. In any batch of bricks which are delivered to your property you may find variations of up to 1/8 inch; and in blocks, hollow tile, and other units, you may find some variations of as much as 1/4 inch or so. These are allowable and should cause you no alarm, because shrinkage is inevitable in drying out or in the baking and varies from batch to batch in the same brick-yard under identical conditions.