The Canadian equivalent to zip codes. Each postal code is made up of six characters, starting with a letter, then alternating between digits and letters. If all combinations were valid, this would allow for about 17 million distinct codes.
However, the codes are constrained somewhat. In particular, the letters 'D', 'F', 'I', 'O', 'Q', and 'U' are never used, presumably because they are too easy to confuse with digits or each other. Additionally, the first letter in a postal code will never be 'W' or 'Z'. This brings the total possibilities down to 7.2 million.
Each letter or digit, starting from left to right, progressively narrows down the geographical region represented by the code. The three characters are called the Forward Sortation Area (FSA). The most significant letter indicates the province or territory. The second character, a digit, determines the urban area; zero is reserved for rural areas. The next letter specifies the postal station.
The last three characters are called the Local Delivery Unit (LDU), and are used to further sort mail within a postal station.
The provinces and their valid inital letters are: