text: Chapter 9
  Figure 9-8. Example of a perimeter defense
  9-61. A major characteristic of a perimeter defense is a secure inner area with most of the combat power
  located on the perimeter. Another characteristic is the ease of access for resupply operations. Units coordinate
  direct and indirect fire plans to prevent accidentally engaging neighboring friendly units and noncombatants.
  Normally, the reserve centrally locates to react to a penetration of the perimeter at any point.
  9-62. Perimeters vary in shape depending on the terrain and situation. Units may reinforce those most
  vulnerable parts of their perimeter with additional resources once they determine the most probable direction
  of enemy attack. The perimeter shape conforms to the terrain features that best use friendly observation and
  fields of fire. Units can increase the effectiveness of a perimeter by tying it into a natural obstacle, such as a
  river, which allows a defending unit to concentrate its combat power in more threatened areas.
  Organization of Forces
  9-63. A perimeter defense has no special organization of forces. They organize in the traditional
  reconnaissance and security force, main body, and reserve. Units may employ all defending forces forward
  along a perimeter or establish a defense in depth within a perimeter. They employ patrols, raids, ambushes,
  air attacks, and supporting fires to harass and destroy enemy forces before they make contact with the
  perimeter, thus providing defense in depth with both techniques.
  9-64. In a forward defense, commanders place all subordinate units in positions along the perimeter. Figure
  9-9 illustrates how a battalion commander could divide the perimeter into subordinate unit assigned areas
  with boundaries and contact points. This method reduces the possibility of fratricide and friendly fire
  incidents within the perimeter and maximizes combat power on the perimeter.
  9-65. Constructing an outer and inner perimeter creating defensive depth is a technique of a defense in depth.
  Figure 9-9 illustrates this technique using an infantry BCT occupying an assembly area while waiting for its
  engineer battalion and third infantry battalion to arrive in the joint operations area. The infantry BCT
  commander directs the two subordinate infantry battalion commanders to place two of their companies along
  the outer perimeter reinforced with detachments from their weapons companies and one company in reserve.
  This configuration gives depth to each battalion’s positions and facilitates control. The figure does not depict
  the cavalry squadron since it occupies positions outside the perimeter in this example. Alternatively, figure
  9-10 illustrates how this infantry BCT commander could elect to assign the outer perimeter to the two infantry
  battalions and have the cavalry squadron resource an inner perimeter, retaining a larger, more tactically
  mobile central reserve.