FM 100-5, Field Service Regulations: Operations, 22 May 1941
Passage of lines is a tactical enabling operation in which one unit moves through another unit's positions with the intent of moving into or out of enemy contact. A commander conducts a passage of lines to continue an attack or conduct a counterattack, retrograde security or main battle forces, and anytime one unit cannot bypass another unit's position. The conduct of a passage of lines potentially involves close combat. It involves transferring the responsibility for an area of operations (AO) between two commanders. That transfer of responsibility usually occurs when roughly two-thirds of the passing force has moved through the passage point. If not directed by higher authority, the unit commanders determine-by mutual agreement-the time to pass command. They disseminate this information to the lowest levels of both organizations.
Units may pass through friendly units during retreat if one flag is rolled. Place the retreating units behind the unit they pass through.
If two or more flags are rolled, it indicates a rout and those units may not pass through friendly line. They take damage for each flag rolled.
Units able to ignore the first flag (for terrain or national characteristics) may pass through friendly lines if two flags are rolled, but are routed if more than two flags are rolled.
Units able to ignore the first and second flag (for terrain and national characteristics) may pass through friendly lines if three flags are rolled, but are routed if more than three flags are rolled.
Units may always move through friendly units during normal movement phase.