chalk
n.— « The following information is placed on a 3×5 inch index card and handed to the pilot by the chalk leader. This serves as a contract between the pilot and the chalk of soldiers to ensure coordination of LZ data. In case the chalk lands in a different LZ, the pilot will write the grid of the new LZ and hand the 3×5 card back to the chalk leader before the soldiers exit the helicopter.» —by Dept. of Army 101st Airborne Division (Air Assault) Gold Book (Fort Campbell, Ky.) Feb. 4, 1999. (source: Double-Tongued Dictionary)
Further reading
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