National System Codes
This is a translator for the "standard" coding patterns in use for the national code system of c.1973-94 with fourteen dot positions. It interprets the inward codes (upper row of code marks), which encode the second part of the postcode (e.g. 5QP). It works out the notation for the dot pattern in standard xx/yy format and states which inward code it corresponds to.
The general idea is that you match the dot pattern on your cover by clicking on the appropriate dot positions to turn them on or off, and the interpretation appears in the box below. The "Start" (S) position was there to provide a fixed point for the machinery to start reading from (working right to left) and a dot is always present in that position. The "Parity" (P) position was for error checking – a dot was printed only when needed to make the number of dots in the full pattern even (so a pattern with an odd number of dots could be assumed to be an error). These positions can be a big help when deciding exactly how many spaces there are between dots ....
The parity dot in this tool will be switched on and off automatically, according to the rest of the pattern.