Key sheet
The daily sheet of Enigma settings that made the machine usable for its own army, and for anyone who intercepted its traffic.
For each day, a key sheet listed the rotor order, ring settings and plugboard pairs to be used on a given network of Enigma machines. Everything that followed depended on it, because it decided how long a stream of messages stayed readable. The Germans distributed the sheets in advance, separately for each network, and the sheets were a prime target for intelligence services. A captured sheet only covered its own day, but several in sequence could reveal a pattern worth working on.