5. At a variable distance in rear of the fire trench (100 to 200 yards) the emplacements for bomb-throwing apparatus and trench mortars are located. These are connected up laterally by a communication trench which joins with the main communication trench running from front to rear through the position. (See Emplacements for Trench Mortars, p. 51). 6. From 100 to 400 yards to the rear of the first line fire trench, and generally parallel to it, is the supporting trench or cover for the supports. This trench is invariably provided with strong overhead cover and a system of dugouts for the protection of the troops. (See Cover for Supports, p. 53). 7. This whole arrangement of trenches is connected throughout from front to rear, and laterally, by a system of zigzag communication trenches. [Illustration: PLATE 1. _PLAN OF THE ORGANIZATION OF AN INTRENCHED POSITION_] Take this brief description together with Plate 1, the drawing that accompanies this volume, and study the two until you get the entire system fixed firmly in your mind; that is, until you get a mental picture of all the elements included in the system. [Illustration: _Organization of a Sector of the Battle Front occupied by a Field Army of Two Divisions._ Plate 2.] After you have done this, study on through the book in order that you may know the purpose of each of these elements and how one links up with the other. 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. 2. Reducing the momentum of the attack, by breaking up the unity of action and cohesion. 3. Holding the enemy under the effective fire of the defenders.