ZIP Codes usually do not have clearly identifiable boundaries, often serve a continually changing area, are changed periodically to meet postal requirements, and do not cover all the land area of the United States. An MA comprises one or more central counties. An MA also may include one or more outlying counties that have close economic and social relationships with the central county. Each state has at least one district. In four States there are one or more cities that are independent of any county organization and thus constitute primary divisions of their States.
States are the primary governmental divisions of the United States.
There are 50 of them, although the census also treats territories and the District of Columbia as equivalents of states for statistical purposes. Micah Altman , Jeff Gill , Gary King , and William Wei .