Departure Parallel. In preparing for an advance upon the enemy, a straight line trench without traverses, and with steps at the end for exit to the surface, is built in front of the first line trench. This line of departure is generally brought within about 200 yards of the hostile line by means of _saps_, short trenches run out from the front line to the new parallel. Since this line of departure can be seen by the enemy, it is sometimes better to construct steps in the front line trench itself, or when possible to build a _Russian sap_. This is a tunnel very near the ground, which can be broken through at a moment's notice when troops are ready to advance. Machine Gun Emplacements.--Shell-holes with a good field of fire and emplacements along boyaux are the best location for machine guns. Few guns are placed in the front line, and these only at strong points in the line, which command a maximum field of effectiveness. Shell-holes may be imitated for machine gun emplacements, but in any case they should be connected by underground passage with the trenches. Thus when trenches are destroyed by bombardment the machine guns remain intact. The field of fire for each machine gun should be carefully determined and marked by three stakes, one for the position of the gun, the other two for the limits of the field of fire. Using these as guides, the gun can be fired correctly at night. During the day it is never left in place nor fired from its actual emplacement. [Illustration: No. 21] Listening Posts.--These stations are usually carried out to the edge of the entanglements nearest the enemy. The listening station must be large enough for half a squad, and often has an automatic rifle and grenade thrower. There should be not more than two posts for each battalion. They are not occupied during the day. They are hard to defend and easily captured by a raiding party which cuts the wire to one of the flanks and comes in from the rear. The boyau leading to the post should be zig-zagged. The post itself should be deep enough for good head cover. Wire Entanglements.--The object of the entanglement is not to stop completely the advance of the enemy, but to delay him at close range under machine gun, auto rifle and rifle fire, and within range of grenades and bombs. Entanglements should be concealed as much as possible from the enemy's trenches and from aerial observation. If possible, they should be placed on a reverse slope or in a dead angle. They should be from 50 to 100 yards in front of the trench, so that artillery fire directed upon the trench will not be effective on the wire. At the same time the wire must not be far enough advanced so that the enemy's raiding parties can cut the entanglements. Wire entanglements are classified as high entanglements, low entanglements and loose wire.