Dugouts were protective holes dug out of the sides of trenches. The size of dugouts varied a great deal and sometimes could house over ten men. A manual published by the British Army recommended dugouts that were between 2 ft. and 4 ft. 6 in. wide, roofed with corrugated iron or brushwood and then covered with a minimum of 9 inches of earth.

As the war went on dugouts grew in size. By 1917 dugouts at Messines could hold two battalions of soldiers at a time. Large dugouts were also built into the side of communication trenches so that they were not directly in line of fire from enemy guns. These often served as the battalion headquarters and provided sleeping accommodation for the officers.