MCRP 12-10B.1, page 89

front along streets, a boundary may have to be extended to one side of the street (Figure 3-7). Figure 3-7. Sector Boundaries in Areas With Semi-detached and Solid-Front Construction (3) Select and Prepare Initial Fighting Positions. Commanders select positions in depth. The unit should prepare positions as soon as they arrive and continue improving positions as long as they are occupied. Enemy infiltration or movement sometimes occurs between and behind friendly positions. Therefore, each position must be organized for all-around defense. The defender should also: (a) Make minimum changes to the outside appearance of buildings where positions are located. (b) Screen or block windows and other openings to keep the enemy from seeing in and tossing in hand grenades. (c) Remove combustible material to limit the danger of fire. Fires are dangerous to defenders and create smoke that could conceal attacking forces. Defenders should store firefighting equipment (water, sand, etc.). The danger of fire also influences the type of ammunition used in the defense. Tracers or incendiary rounds should be avoided if the threat of fire exists. (d) Turn off electricity and gas. Both propane and natural gas are explosive. Natural gas is also poisonous and is not filtered out by a protective mask. Propane gas, although not poisonous, is heavier than air. If it leaks into an enclosed area, it displaces the oxygen and can cause suffocation. Gas mains and electricity should be shut off at the main distribution nodes. (e) Battle positions should be located so as not to establish a pattern. The unit should avoid obvious firing locations like church steeples. (f) The unit should rehearse movements between positions. (g) Camouflage positions. (h) Reinforce positions with available materials, such as beds, furniture, etc. (i) Block stairwells and doors with wire or other material to prevent or delay enemy movement. Create holes between floors and rooms to allow for covered movement within a building. (j) Prepare range cards, fire plans, and sector sketches. MCWP 3-35 .3 3-18