CPC Binary is one of the bar code systems used by Canada Post for mail sorting, along with PostBar. While PostBar is printed in normal black ink like most other bar codes, CPC Binary is printed in a special ultraviolet-fluorescent ink. The bars in CPC Binary don't differ from one another in width or height as in other systems, but represent values depending on their position relative to spaces, so that bars and spaces can be regarded like ones and zeroes in a binary code system. Special constraints are applied to the code system so that there aren't an excessive number of spaces in a row, as these would be hard to count. The number of bars in a row without a space are also limited.