ANSI INCITS-31-2009 pdf download Codes for the Identification of Counties and Equivalent Areas of the United States, Puerto Rico, and the Insular Areas
4. Assignment of identifying data codes
4.1 Assignment of county codes Each county and county equivalent to be identified under this standard shall be assigned a single county code consisting of three digits. Each assigned county code shall be unique within a state or state equivalent.
4.2 Initial sequencing of names for county code assignment For each state and state equivalent, the names of the counties and county equivalents (except for independent cities) shall be arranged in alphabetical order. This sequence shall be followed by the names of the independent cities of that state or state equivalent, if any, also arranged in alphabetical order.
4.3 Initial assignment procedure For each state and state equivalent, county codes shall be assigned in an ascending numerical sequence to the county and county equivalent names arranged as specified in 4.2. One and only one county code shall be assigned to each county or county equivalent. 4.4 Assignment interval and values for county codes In the initial assignment of county codes within each state, there shall be a minimum arithmetic interval of two between successive assignments. For each state except Alaska, and for the District of Columbia and Puerto Rico, the lowest numbered initial assignment shall be 001 . For Alaska, the lowest numbered initial assignment shall be 01 0. For independent cities of any state or state equivalent, the lowest numbered initial assignment shall be 51 0.
4.5 New entries Subsequent to the initial assignments, new entries shall be assigned county codes so that, to the greatest extent possible, the alphabetic order within each sequence is maintained.
4.6 Unique representation To permit unique representation of each county or county equivalent among all such units identified under this standard, the three-digit county code shall be used in conjunction with a two-character representation of the state or state equivalent in which the county or county equivalent is located. The two-character representation of each state and the District of Columbia shall be specified in codes maintained at http://www.census.gov/geo/www/ansi/ansi.html as detailed in INCITS 38. The alphabetic representations of state equivalents other than the District of Columbia shall be those for which the U.S. Post Office provides a structure