Opus quadratum

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Opus quadratum, or "squared work," is an ancient Roman building method using square stone blocks cut to the same height and laid in level horizontal layers. The blocks are placed side by side, usually without mortar. The technique is described by Vitruvius. It began around the 6th century BC and became more precise over time. It continued through the Roman era and was often used alongside other methods as mortar appeared. The stone type, block size, and how the joints fit together help date buildings that use this technique. In the early form (the Etruscan way), joints were visible and blocks could be uneven. In the later "Greek way," blocks were placed in two rotations: "stretchers," with the long side facing the wall, and "headers," with the short side facing the wall and extending farther back into the wall. Different patterns could be created by changing placement, and joints were often centered over the blocks below for strength. With Roman concrete, outer walls could be continuous, sometimes using headers to connect to the inner wall. Tile or marble might be bonded to these walls, but this was rarer for very heavy, load-bearing parts like arches, bridges, and aqueducts.