The United States Postal Service uses two-letter abbreviations for the U.S. states and territories and former territories of the US, as specified below. For other purposes as well, they are frequently used. Canada Post also recognizes these postal abbreviations.
The following abbreviations for the Canadian provinces and territories, used by Canada Post, are also recognized:
Almost all of these are also used in domain names, such as vancouver.bc.ca and seattle.wa.us; however, the Yukon domain is .yk.ca
Other abbreviations used by the US postal service:
The USPS recommends the address be written in block capital letters, probably for better processing by their OCR machinery. They prefer some standard abbreviations for common terms in street addresses.