for enciphering; the operator sets the enciphering-deciphering knob to "decipher", and aligns the key wheels to the same sequence as was used in enciphering. The first letter of the ciphertext is entered via the indicator disk, and the power handle is operated, advancing the key wheels and printing the decoded letter on the paper tape. When the letter "Z" is encountered, a cam causes a blank space to appear in the message, thus reconstituting the original message with spaces. Absent "Z"s can typically be interpreted by the operator, based on context. An experienced M-209 operator might spend two to four seconds enciphering or deciphering each letter. Internal elements Overview Inside the casing of the M-209, a much more complicated picture emerges. The six key wheels each have a small movable pin aligned with each letter on the wheel. These pins may each be positioned to the left or right; the positioning of these pins affects the operation of the machine. The left position is ineffective, while the right position is effective. Each key wheel contains a different number of letters, and a correspondingly different number of pins. From left to right, the wheels have: 26 letters, from A to Z 25 letters, from A to Z, excepting W 23 letters, from A to X, excepting W 21 letters, from A to U 19 letters, from A to S 17 letters, from A to Q This discrepancy is chosen to give the wheel sizes a coprime nature; the end result is that the wheels only align the same way once every 26×25×23×21×19×17 = 101,405,850 enciphered letters (also known as the period). Each key wheel is associated with a slanted metal guide arm that is activated by any pins in the "effective" position. The positions of the pins on each key wheel comprise the first part of the internal keying mechanism of the M-209. Behind the row of six key wheels is a cylindrical drum consisting of 27 horizontal bars. Each drum bar is affixed with two movable lugs; the lugs can be aligned with any of the six key wheels, or may be placed in one of two "neutral" positions. An effective pin causes its guide arm to tilt forward, contacting the drum. The positioning of the lugs comprises the second part of the internal keying mechanism. Owing to the complexity of setting the internal keying mechanism, it was altered relatively