Zip codes are used by the US Postal Service to make mail delivery more efficient. The basic format of US zip codes consists of five numbers. The first three numbers represent the sectional center facility and the other digits represent the older postal code for a specific state or city. The process of using postal coded for US cities came into effect in 1943, but in 1963, zip codes came into use for the whole country.

The sectional center facility part of the zip code is the center for processing the mail. Here the mail is sorted according to the first three digits of the zip code and sent to the respective centers in each location. These digits identify the centers all over the country. In the individual centers, the mail is then sorted according to the last three digits and sent to the post offices where the addressees pick up their mail. Sectional centers do not deliver mail and are not open to the public.

Up until 1967, second and third class mail did not require a zip code. After that date however, it became mandatory for all mail to have the proper zip code in the address. In 1983, the US further expanded the zip code feature making it the Zip Code + 4. These four digits were then used to identify a specific geographical area. In the SCF, a machine identifies the zip code and sorts the mail so that it goes to the correct post office.

Today the first digits of the zip code identify the state or SCF. The second and third digits identify a region of the state and the fourth and fifth numbers identify a specific address in that region. The main town gets the zip code and then the surrounding towns follow in numerical order.

The allocation of the first number in the zip code is as follows:

- 0 - Connecticut, Massachusetts, Maine, New Hampshire, New Jersey, Puerto Rico, Rhode Island, Vermont, US Virgin Islands, Army Post Europe, Fleet Post Office Europe

- 1 - Delaware, Pennsylvania and New York

- 2 - District of Columbia, Maryland, North Carolina, South Carolina, Virginia, and West Virginia

- 3 - Alabama, Florida, Georgia, Mississippi, Tennessee, Army Post Office Americas, Fleet Post Office Americas

- 4 - Indiana, Kentucky, Michigan and Ohio

- 5 - Iowa, Minnesota, Montana, Montana, North Dakota, South Dakota and Wisconsin

- 6 - Illinois, Kansas, Missouri and Nebraska

- 7 - Arkansas, Oklahoma, Texas and Louisiana

- 8 - Arizona, Colorado, Idaho, New Mexico, Nevada, Utah and Wyoming

- 9 - Alaska, American Samoa, California, Guam, Hawaii, Marshall Islands, Federated States of Micronesia, Northern Mariana Islands, Oregon, Palau, Washington, Army Post Office Pacific, and the Fleet Post Office Pacific.

The zip codes can also be divided and changed when a rural area becomes a suburb of a larger city. The new codes become effective once they are announced, but there is a grace period to allow the residents to get used to their new zip code. When a city expands so that the sectional facility cannot handle all the mail coming through it, it is sometimes necessary to open a new facility, which then receives its own number. This also changes the zip code for the people living in that area.