Cardboard slide ruler. The sliding element was a line cut from a monthly page, with a line for each day of the month, and an up/down arrow on the right side.
The cipher operator aligned the arrow with the letter indicating the OTP (One Time Pad) network symbol, and randomly chose three letters (for example: LUL), and composed the five letter group, used as code indicator (example: LULUL).
The slide ruler in the photo has 26 letters. IDF also used a 25 characters ruler (the letter V was omitted) and a double length ruler (50 characters).
Used by the IDF during the War of Independence (1948) and the Sinai Campaign (1956).
