This is the typical system now in use in the European war theaters. Circumstances at certain places may render some variations necessary, and it must not be inferred that the trace of the works is the same throughout. As a rule the types of trenches (altered when necessary to meet local conditions) illustrated herein are the ones in actual use on the war fronts.
All of these trenches and their accessories constitute what is known as the first line. At a distance of from 2,000 to 5,000 yards in rear of this first line a second line, organized in a similar manner, is to be found.
At intervals of from 800 to 1,500 yards along the first line-centers of resistance, or what we know as "supporting points," are located. These consist of fortified villages, or a network (labyrinth) of trenches, provided with every defensive device known to modern warfare. The object of these supporting points is to bring a flanking fire to bear on the intervals between them, with the idea that an attacking force cannot advance beyond them without capturing them.
Plate 2 shows the general scheme of the occupation of a sector of the line by a field army of two divisions.
Chapter II
OBSTACLES
The element of the defensive line nearest the enemy is a line or series of lines of obstacles which are designed for the purpose of:
1. Protecting the lines from surprise.