Long Tao: Yīn Shū (Secret Documents)

King Wu asked Tai Gong:
"When leading troops deep into enemy territory, if the sovereign and commander wish to mobilize forces, adapt flexibly to enemy movements, and achieve unexpected victories—yet face intricate affairs too complex for tallies to convey, with vast distances rendering verbal communication impossible—what should be done?"

Tai Gong replied:
"For grand strategies and covert plans, use secret letters (yin shu), not tallies. The sovereign sends instructions to the commander via secret letters, and the commander petitions the sovereign through the same. These letters are ‘split once and divided thrice, dispatched in three parts but understood as one.’

To ‘split once and divided thrice’ means dividing a single message into three sections. To ‘dispatch in three parts but understood as one’ means sending three couriers, each carrying one fragment. The fragments overlap and scatter, ensuring even the couriers cannot grasp the full meaning—this is the essence of yin shu. Thus, no matter how cunning the enemy, they cannot unravel the secret."

King Wu said:
"Brilliant!"