The sides of the trench are kept as nearly perpendicular as possible, to give the maximum protection from shell burst and the fall of high angle projectiles. The _parados_, the bank of earth to the rear of the trench, has been developed during the war to give protection from flying fragments of shells exploding to the rear, and to prevent the figure of a sentinel from being outlined through a loop-hole against the sky. The _berm_, a ledge or shelf left between the side of the trench and the beginning of the parados, has come into general use in order to take the weight of the parados off the earth at the immediate edge of the trench, and so prevent the reverse slope from caving in easily under bombardment or heavy rain. [Illustration: Plate #16A] Automatic guns have made it necessary to break the line of the trench at every opportunity, in order to secure a flanking fire for these arms. Auto-rifles and machine guns have tremendous effectiveness only in depth, and flanking fire gives them their greatest opportunity. Trench Construction.--The methods of building trenches are the same whether the work is carried on under fire or not. In an attack, upon reaching the limit of advance, the men immediately dig themselves in, and later connect these individual holes to make a continuous line of trench. Most of the digging must be done at night, and must be organized to obtain the most work with the least confusion. There are three ways of increasing the efficiency of the men. In the first of these, squad shifts, the squad leader divides his men into reliefs and gives each man a limited period of intensive work. Reliefs may be made by squads or by individuals. The second way of increasing efficiency is to induce competition among the man and squads, thus making the work a game in which each soldier's interest will be aroused in the effort to do better than the others. The third method is to assign a fixed amount of work to each man. An average task, which all ought to accomplish in a given time, is found by experience, and those who finish before their time is up are relieved from further work during that shift, and allowed to return to their shelters. Continual care must be used to check up the tools on hand, as the men are prone to leave them where they were working rather than carry them back and forth to work. Each unit must guard its property from appropriation by neighbors on its flanks. System of Laying Out Trenches.--The trace of the trench is first staked out, particularly at traverses and corners when the work is to be done at night. Measurements should be exact, and the men should be required to line the limits of each trench so as not to exceed them in digging. All sod should be taken up carefully and used on the parapet for concealment or on the berm to make a square back wall for the dirt of the parapet. If possible this should be done with the parados wall, so as to make it as inconspicuous as possible from the front.