1. Be close to the defender’s position. As a rule on the western front they are not more than from 50 to 100 yards distant. If they are too close it may be possible to throw hand grenades from the far edge of them into the defender’s trenches. 2. As far as practicable, be sheltered and screened from the enemy. Shell fire is the most effective method of destroying obstacles. If they are not concealed they may furnish aiming points for the enemy’s fire against the first line fire trench by his being able to estimate its location with reference to the obstacle. 3. Afford no cover or screen to the enemy. 4. Be so placed that the enemy will come upon them as a surprise. 5. Be so constructed as to be difficult of removal under fire and impracticable to negotiate while still reasonably intact. 6. Be arranged so as not to interfere with a counter attack. The obstacles may have occasional gaps left in them which may be mined. The different classes of obstacles are: Abatis, low wire entanglements, high wire entanglements, barricades, mines, fougasses, crows feet, military pits with wire entanglements, inundations, etc. [Illustration: PLATE 3.—Abatis.] Abatis (pronounced _abatee_) consists of branches of trees lying parallel to each other, butts pointing to the rear, and the branches interlaced with barbed wire. All leaves and small twigs should be removed and the stiff ends of branches pointed. The butts are staked or tied down of, anchored by covering them with earth. When more than one row is used the branches overlap the butts of those in front so as to make the abatis about 5 feet high. An abatis formed by felling trees towards the enemy, leaving the butt hanging to the stump, is called _slashing_. [Illustration: PLATE 4.—Slashing.] [Sidenote: Wire Entanglements] Barbed wire is the material most employed in the construction of obstacles. It may be used in the following manner: 1. As a simple trip, for giving the alarm. It is stretched just above the ground and attached to some object that will cause a noise to be made if molested. 2. A simple wire fence, to cause delay and confusion to the enemy in his advance. 3. As an adjunct to tree and brushwood entanglement. 4. As a wire entanglement. 5. As a covering for portable cylinders. The advantages of the barbed wire entanglement are: 1. It is easily and quickly made, 2. It is difficult to destroy. 3. It is difficult to get through. 4. It offers no obstruction to the view and fire of the defense. The low wire entanglement is constructed as follows: 1. Drive stakes in the ground until they project about 18 inches. The stakes should be about 6 feet apart, those in each row being opposite the intervals in adjacent rows, 2. The wire is then passed loosely from the head of one stake to another, wound around each and stapled. 3. Where two or more wires cross they should be tied together.