Prior to the use of zip codes, cities were divided into regions and mail was addressed as Rochester 10, New York, for example, to show which region of the city the mail was intended for. As cities continued to grow this system was not sufficient and was replaced by the Zip Code. ZipCodes went into effect in 1963. The United States Postal Service (USPS) initially used a cartoon character Mr. Zip to promote the use of zip codes. In 1983 the zipcode added the last four digits in an attempt to more narrowly define an address.
The first three digits of a zip code indicate the sectional center facility which will ultimately sort and distribute that piece of mail. All mail to that 5 digit zipcode will first arrive at the sectional center facility then after sorting continue along to its exact destination.
Since the first digit of Rochester's zip code is 1 it is located in the "1" zone that includes the following areas: Delaware, New York, and Pennsylvania.
