The arrangement of bricks in distinctive courses in a wall. As continuous vertical joints are structurally unsound, various types of bond were evolved. The normal method in the Middle Ages was the English bond, by which courses were arranged alternately in stretchers (bricks laid at length along the face of the wall) and headers (bricks laid at right angles to the face of the wall). Flemish bond (alternate stretchers and headers in the same course) became popular in the 17th century. There are several variations on these two basic types.