A =sap= is a narrow trench (subsequently widened), which is continually prolonged in the desired direction by digging away the earth at its head and throwing it to the front and exposed flank as a cover for the working party. When the sap is subject to an oblique front-fire, exposing one flank only, the parapet is constructed on that flank and at the head. This is known as a _single_ or _full sap_. When both flanks and the head of the sap are exposed to fire two full saps are driven parallel and very near to each other, each with its parapet on the outer flank. The tongue of earth left between them is removed to widen the narrow trenches, thus making a single trench with a parapet on each side. This is called a “_double sap_.” The trench is sometimes deepened and given a splinter-proof roof or cover. This is known as a “_blinded sap_.” A sap gaining ground to the right and front is called a “_right-handed sap_;” its parapet is on its left flank. A “_left-handed sap_” has its parapet on its right flank. To expedite the work in sapping several reliefs should be employed, and task-work should be adopted to induce the men to work rapidly. In all sapping operations the use of the simple trench and flying sap will be resumed when circumstances admit without involving too great losses. =23. The full sap= (Pl. II, Figs. 17-21) requires a detachment, or “=brigade=,” of 1 non-commissioned officer and 8 men, provided with the following tools, viz.: For No. 1, a miner’s pick, a miner’s shovel, a measuring-rod, 4' 6", marked at 3', and a sand-bag fork. No. 2, a measuring-rod of 1' 6" and a shovel. No. 3, a pick, a shovel, and a measuring-rod 5' long, marked at 4' 6". No. 4, a shovel and a scraper with a handle 9' long. For the rest of the detachment, a 6' measuring-rod, knee-caps for 4 men, 2 shovels and 1 pick (in reserve), and, when necessary, a crowbar, axe, and bill-hook. From 100 to 150 sand-bags are supplied to each detachment. =24. Organization and Duties of the Detachment.=--The sappers are numbered 1, 2, 3, and 4 in each rank; the front rank extends the sap 1 yard and is then relieved by the rear rank, and so alternately. The sappers change places at each relief; those who serve as Nos. 1 and 2 during their first task becoming Nos. 3 and 4 during the second, and so on throughout their tour. If a detachment is reduced below 8 in number by casualties it nevertheless keeps 4 men at work driving the sap, and reduces its reserve until new men are supplied.