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.
2. Reducing the momentum of the attack, by breaking up the unity of action and cohesion.