Communication Trench (or Boyau).—­Running to the rear and joining the different parallel resistance trenches are communication trenches or boyaux.  These are for transportation of men and material as well as for communication.  Communication trenches should be made wide enough to allow travel for men with loads, should be at least seven feet deep and as smooth as possible on the bottom.  Rough places will delay traffic.  They are dug with turns every 20 or 30 yards to prevent their being swept by gun fire.  When boyaux are built by night sharp zig-zag corners should be used, or the angles will not be acute and protection will be lost.  During daylight when the directions can be seen, the construction may be a serpentine curve, with no stretch of more than 30 yards visible from one point.

Boyaux are sometimes used for lateral defence and often emplacements for automatic guns are arranged to cover stretches of them.  Bombing stations are placed near by to protect the guns and to clear the boyau of the enemy.  At these points the boyau is left straight for a short distance in front.  Where provision is made for lateral or frontal defence by rifle fire, firing steps are constructed.  If this is inconvenient for movement along the boyau, individual emplacements must be made in the side wall for firing.  Sentry posts are dug at right angles to the boyau.

Arrangements for passage of men moving in opposite directions may be made by extending short spurs at the corners, enlarging the boyau at the bends, digging niches or passing points here and there, or constructing island traverses with the boyau running around on each side.

[Illustration:  Plate #19]

Every boyau should be marked where it meets a trench with a sign indicating the place and trench.  Without this messengers, reliefs and re-enforcements may easily be lost in the maze of trenches.

Latrines should be run out about 20 feet from boyaux at points directly in rear of lateral trenches.  If possible they should be placed so that men cannot enter them without passing near the platoon leaders.  This will prevent men from leaving the front line, under the pretense of going to latrines, during bombardments or mining operations.  The trench leading to a latrine should be constructed like a boyau, and the pit should be close to the side nearest the enemy, to give the best possible protection from shell fire.  There are three types of trench latrines:  Deep boxes which are covered and have rough seats; short straddle trenches or trenches equipped with a single horizontal bar, and portable cans, used where the ground or the limited space make it impossible to dig pits.  These cans should be emptied daily into holes behind the trenches, which are covered after the cans are emptied into them.

Urinals should be separate from the latrines.  They may be either holes about three feet deep filled with stone, troughs with a covered pit at the end, or portable cans.