No two letters would be enciphered using the same substitution alphabet— until , that is, the bicycle worked its way back around to the initial state ([theta] = 0, C = 0) and began to repeat the cycle. This means that it is a periodic polyalphabetic system. Now, if this machine had a short period, it would repeat itself frequently, and would therefore be useful, as an encryption system, only against kids in treehouses. The longer its period (the more relative primeness is built into it) the less frequently it cycles back to the same substitution alphabet, and the more secure it is.