[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.

High Entanglements are strung on metal or wood posts about four feet high, both wire and posts being painted for camouflage.  The driving of posts must be muffled, and metal screw posts are used when near the enemy’s line.  Posts are placed in two parallel lines, two yards apart, spaced alternately so that the posts in one line are opposite the middle of the interval in the other.

Tracing Entanglements is done by a sergeant followed by two stake placers, two holders and two drivers, who in turn are followed by men attaching wire.  Two men carry each roll of wire, and each pair (there are twelve pairs in all) strings one wire.  A panel between stakes is composed of four strands.  Each wire should be wrapped around each post.  The same arrangement of panels should run between the two lines of posts.  The entanglements are in three lines about 20 yards apart, the nearest being 20 yards from the front line trench.  The entanglement nearest the enemy should be constructed first, so that men always work nearest their own trenches.  All wire entanglements should be at all points commanded by the flanking fire of machine guns.  High entanglements (known as abatis) may be made by felling trees toward the enemy, and similar entanglements made of brushwood are useful in emergency.

Low Entanglements are formed of pickets two feet high, 2-1/2 inches in diameter, wired in all directions.  Vegetation renders the entanglement invisible from the enemy and from aerial observation.  This type may also be placed in shallow excavations which are concealed from the enemy and partly protected from artillery fire.  Sharpened stakes, with their points hardened by fire, driven obliquely into the ground, may also be used.