In an attack all boyaux are used for movements from rear to front.
All movements to the rear while reinforcements are being brought up must be executed in the open ground, exceptions being made in the case of wounded.
Boyaux are important for the movement of supplies and ammunition, but above all they are essential for reinforcements during battle. The greater part of the troops in a sector is held in reserve in the second and third lines. This disposition is only possible if you have good boyaux to the front lines.
All boyaux and communicating ditches are narrow and deep. Boyaux are wider between the second and third line than they are between the first and second. Evacuation boyaux are usually wider than others. The trace of a boyau must be such as to prevent enfilade fire.
At intervals of about ten yards turnouts large enough for a stretcher are constructed in the side of the boyaux for passage of troops in different directions. Boyaux are named and signs are put up similar to those for streets in a city. The same name of a boyau must be kept from the third lines to the first.
At branches of the main boyaux they are dug a little deeper to avoid mistakes of identification.
As was said before certain boyaux are organized for defense.
It may be said here that supplies carried up at night are usually brought along on top of the ground close to the boyaux. This gives freer movement and the carriers are close to the cover of the trench if needed.
Accessory Defense. Of all the accessory defenses such as barricades, abatis, trous-de-loup, etc., the most important by far is barbed wire, which is almost exclusively used for obstacles.
Barbed wire should be placed from twenty to fifty yards in front of the first line. At this distance shelling of the barbed wire belts will do very little damage to the first line.