Treatise on Armament Technology, Volume 11: Formation and Training Regulations — Issuing Orders II

Within the camp, a specific officer shall be appointed to manage the night password ledger. In issuing passwords, it is forbidden to repeat them, for fear of confusion. The password must be personally issued by the General as night falls; it must not be fixed in advance. "Fixed in advance" refers to the danger of setting passwords days ahead of time, which increases the risk of the enemy discovering the sequence.

In a field camp, if a spy Jianxi — an infiltrator or enemy agent is found to have slipped into the camp during the night, simply order the troops not to move or shout. They must maintain solemn silence and refrain from making noise. All units should remain seated; then the spy will naturally not dare to stir. Wait until dawn to identify them; anyone who moves is the enemy. By forcing everyone to remain perfectly still and silent in their designated positions, an outsider who does not know the specific seating arrangement or who attempts to move in the dark becomes immediately conspicuous.

When a field camp is located near the enemy's fortifications,
soldiers often sit in the dark through the five watches of the night. If one wishes to track the timing of the watches, it is necessary to use the "passing of arrows" as a standard within a radius of several miles around the camp. For example, if fifty rotations of the arrow occur from the first watch until dawn, then each watch consists of ten rotations. This is used as original: "传箭" (Chuanjian). This was a traditional method of timekeeping where an arrow or tally was passed from soldier to soldier around the camp perimeter. By counting how many times the arrow completed a full circuit, the commander could calculate the passage of time without relying on water clocks, which were difficult to use in the field.