The device consisted of 25 aluminium discs attached to a four-and-a-half inch long rod, each disc containing the 26 letters of the Roman alphabet in scrambled order around its circumference (with the exception of the 17th disc, which began with the letters "ARMY OF THE US"). Each wheel had a different arrangement of the alphabet, and was stamped with an identifying number and letter; wheels were identified according to the letter following "A" on that wheel, from "B 1" to "Z 25". The wheels could be assembled on the rod in any order; the ordering used during encoding comprised the key. There were "25! (25 factorial) = 15,511,210,043,330,985,984,000,000" (more than 15 septillion) possible keys, which can be expressed as about an 84-bit key size.