text: Chapter 6
  when each element is relieved in a sequence determined by the tactical situation, not its geographical
  orientation. Simultaneous relief takes the least time to execute but is more easily detected by enemy forces.
  Sequential or staggered reliefs can occur over a significant amount of time. These three relief techniques can
  occur regardless of the operational theme in which the unit is participating. (See ADP 3-90 for more
  information about relief in place.)
  PASSAGE OF LINES
  6-159. A passage of lines is an operation in which a force moves forward or rearward through another
  force’s combat positions with the intention of moving into or out of contact with the enemy (JP 3-18). There
  are two types: a forward passage of lines and a rearward passage of lines. A forward passage of lines occurs
  when a unit passes through another unit’s positions while moving toward the enemy (ADP 3-90). A rearward
  passage of lines occurs when a unit passes through another unit’s positions while moving away from the
  enemy (ADP 3-90).
  6-160. Units perform a passage of lines to continue their attacks or perform counterattack, retrograde, and
  security tasks that involve advancing or withdrawing through other units’ positions. A passage of lines
  potentially involves close combat. It involves transferring the responsibility for an AO between two units.
  That transfer of authority usually occurs when roughly two thirds of the passing force have moved through
  one or more passage points. The headquarters directing the passage of lines is responsible for determining
  when the passage of lines starts and ends. If not directed by higher authority, the stationary unit commander
  and the passing unit commanders determine—by mutual agreement—the time to pass responsibility for an
  area. They disseminate this information to the lowest levels of both organizations.
  6-161. Units have several reasons for conducting a passage of lines. These reasons include to—
  (cid:122) Sustain offensive tempo and avoid culmination.
  (cid:122) Maintain the viability of the defense by transferring responsibility from one unit to another.
  (cid:122) Transition from the performance of delay or security tasks by one force to a defending unit.
  (cid:122) Relieve a unit for another mission or task.
  (See ADP 3-90 for more information on passage of lines.)
  COUNTERMOBILITY OPERATIONS
  6-162. Countermobility is a set of combined arms activities that use or enhance the effects of natural and
  man-made obstacles to prevent the enemy freedom of movement and maneuver (ATP 3-90.8). The primary
  purposes of countermobility are to shape enemy movement and maneuver and to prevent the enemy from
  gaining a position of advantage. Countermobility is conducted to support forces operating along the range of
  military operations. Countermobility directly supports offensive and defensive operations.
  6-163. In support of offensive operations, countermobility activities isolate objectives and prevent enemy
  forces from repositioning, reinforcing, and counterattacking. They enable flank protection as operations
  progress into the depth of enemy defenses or as an integrated economy-of-force effort to provide general
  flank security. In support of defensive operations, countermobility activities disrupt enemy attack formations
  and assist friendly forces in defeating the enemy in detail, to channel attacking enemy forces into an
  engagement area or areas throughout the depth of the defense, and to protect the flanks of friendly
  counterattack forces.
  6-164. The conduct of countermobility activities typically requires engineers and includes proper obstacle
  integration with the maneuver plan, adherence to obstacle emplacement authority, and positive obstacle
  control. Combined arms obstacle integration synchronizes countermobility into the concept of operations.
  Because most obstacles have the potential to prevent the freedom of movement and maneuver to both friendly
  and enemy forces, it is critical that commanders weigh the risks and evaluate the trade-offs of employing
  different types of obstacles. Countermobility activities include—
  (cid:122) Siting obstacles.
  (cid:122) Constructing, emplacing, or detonating obstacles.
  (cid:122) Marking, reporting, and recording obstacles.
  (cid:122) Maintaining obstacle integration.