infrequently; changing internal keys once a day was common in practice. When the operator turns the power handle, the cylindrical drum makes a complete revolution through all 27 bars. If a lug on one of the bars contacts the guide arm of an active key wheel, that bar is slid to the left; lugs in neutral positions, or which do not contact a guide arm, do not affect the position of the bar. All bars that are slid to the left comprise a variable-toothed gear, which in turn shifts the letter to be encoded; the shift is equal to the number of bars protruding to the left. The resulting ciphertext letter is printed onto the paper tape. After the rotation is complete, a retractor pushes the protruding bars back into place. A set of intermediate gears advances the key wheels by one position, and a locking arm latches into the drum to prevent a second encoding until the indicator disk is adjusted for the next letter. This system allowed the offset to change for each enciphered letter; without this facility, the enciphering scheme would resemble a very insecure Caesar shift cipher. Example configuration Prior to encoding anything using the M-209, the operator must set the machine according to a preset configuration. This configuration includes the settings for each pin on all six of the key wheels, and the position of each lug on the rotating drum; these were typically specified by tables in a secret system publication given to both sender and receiver. The rotational alignment of the key wheels could be chosen by the sender at random, and provided to the receiver via a secure channel of communication. Each letter on each key wheel is associated with a pin that can be set either to the left or right. A table specifying the setting of these pins might resemble the following: Letters that are present in the table for a given key wheel should have their corresponding pin set to the right, or "effective", position. Absent letters, represented by a dash, are set to the left, or "ineffective", position. The rotating drum has 27 bars, each with two lugs. These lugs can be set to any position 1 through 6, in which case they are aligned with the corresponding key wheel, or they may be set to one of two "0" positions, in which case they are ineffective. A