In the normal case there will be some natural cover available. Such, however, is not always the case, and specially constructed observation stations have to be provided. The posts should be placed in advance of the first line trench, the distance depending upon circumstances which have to be determined in each particular instance. They must be fully protected from reverse fire so that there will be no chance of the observer masking the fire of his comrades manning the fire trench. Unless the ground is very favorable it will be found difficult to provide for observation above ground. Where there are natural features such as embankments, mounds, hedgerows, ruins of buildings, etc., it may be possible to make provision for observation even by day. Where a loophole is used, the type having the narrower end outward should be provided. In the open type of post the observation directly to the front may be greatly facilitated by the use of the periscope. (Plate 11.) A good, strong parapet thrown up and chopped off at the corners will enable the observer to cover areas from an oblique direction from the post and protect him from fire from the front. In the covered type the observer is provided loopholes having the splay towards him. These may also be constructed to the oblique rather than to the front. When this is done, provision must be made to cover the entire front of the position from the several posts. (Plate 12.) [Illustration: PLATE 11.—Open type of listening post.] [Illustration: PLATE 12.—Covered type of listening post.] [Illustration: PLATE 13.—A listening and observation post.] The post may be connected with the first line fire trench by a narrow zigzag trench or by an underground passage. If the former, it must be thoroughly concealed and have no excavated earth visible. If it can be located along a hedge or some other natural feature its location may remain unknown to the enemy for a considerable length of time. Where a communication gallery is constructed the roof and walls must be suitably shored up by casing and supports. The sentinel in the listening post carries no accouterments. It has been found that the creaking noise made by equipment when the sentinel moves has been taken for mining operations of the enemy by his comrades. Listening galleries should never be left without a sentinel. There should be a depot of arms and hand grenades near the entrance to the gallery in case men are attacked while on duty from either above or below ground. Listening will be conducted at specified times, or on some prearranged signal, and for a definite period. During this time all within the listening area, including the trenches, must remain absolutely motionless. Infantry manning a trench can assist listening by digging a small pit, 6 feet deep below the trench, and running a bore-hole out 20 feet or more.