The precursor to our modern zip code system began in the 1940's. Codes were first used in large cities in order to help quicken mail distribution. The volume of mail increased dramatically after WWII and the US Post Office needed to adopt a new system nationwide. Zip codes became mandatory for 2nd and 3rd class mail and bulk rate mail in 1967. The zip code system expanded in the 1980's to include +4. The +4 zip code is required for bulk mail.