General Arrangement.—­A position is a combination of trenches, consisting of:  The fire trench, or first line, nearest the enemy; the cover trench, just behind the first line, where all but sentinels of the fire trench garrison are held in dugouts or shelters; the support trench, from 150 to 200 yards in rear of the cover trench, and the reserve, from 800 to 1,200 yards still further to the rear.

The support trench is placed far enough from the first line to prevent the enemy from shelling both trenches at once.  By a concentration of artillery fire and a determined advance of the hostile infantry the first line may be captured.  The support trench must be so organized that it will then act as a line of resistance upon which the enemy’s advance will break.  Lieutenant Colonel Azan of the French army says:  “As long as the support trenches are strongly held, the position is not in the hands of the enemy.”

[Illustration:  Plate #16]

The reserve is usually a strong point, so organized that it can maintain independent resistance for several days if necessary, should the enemy obtain control of adjacent areas.

Where possible trenches should be on reverse slopes, with the exception of the first line; but usually the outline of a trench is determined in actual combat, or is a part of hostile trench converted.  Under these circumstances it cannot be arranged according to tactical ideals.

Artillery and the automatic gun are the determining factors in trench warfare to-day.  The effect of artillery fire must be limited in its area as far as possible, and trenches are, therefore, cut by traverses, which are square blocks of earth not less than nine feet square, left every 27 feet along the trench.  They should overlap the width of the trench by at least one yard, thereby limiting the effect of shell burst to a single bay, the 27-foot length of firing trench between two traverses.  Sharp angles have the same effect as traverses, but angles of more than 120 degrees cannot be utilized in this way.

The sides of the trench are kept as nearly perpendicular as possible, to give the maximum protection from shell burst and the fall of high angle projectiles.  The parados, the bank of earth to the rear of the trench, has been developed during the war to give protection from flying fragments of shells exploding to the rear, and to prevent the figure of a sentinel from being outlined through a loop-hole against the sky.  The berm, a ledge or shelf left between the side of the trench and the beginning of the parados, has come into general use in order to take the weight of the parados off the earth at the immediate edge of the trench, and so prevent the reverse slope from caving in easily under bombardment or heavy rain.

[Illustration:  Plate #16A]

Automatic guns have made it necessary to break the line of the trench at every opportunity, in order to secure a flanking fire for these arms.  Auto-rifles and machine guns have tremendous effectiveness only in depth, and flanking fire gives them their greatest opportunity.