Opus spicatum

Its usage was generally decorative and most commonly it served as a pavement, though it was also used as an infill pattern in walls, as in the striking base of the causeway leading up to the gate tower at Tamworth Castle. Unless the elements run horizontally and vertically, it is inherently weak, since the oblique angles of the elements tend to spread the pattern horizontally under compression. The type of construction was constantly employed in Roman, Byzantine and Romanesque work, and in the latter was regarded as a test of very early date. It is frequently found in the Byzantine walls in Asia Minor, and in Byzantine churches was employed decoratively to give variety to the wall surface. Sometimes the diagonal courses are reversed one above the other. The herringbone pattern produces opposing shear plane faces, increasing the relative surface area and therefore rendering it a sounder design for mortar and brick. Herringbone work, particularly in stone, is also used to make firebacks in stone hearths. Acidic flue gases tend to corrode lime mortar, so a finely set herringbone could remain intact with a minimum of mortar used.