states and countries

1. States

In text, consistently spell out states' names rather than using U.S. Postal Service abbreviations.

California (rather than CA)
Colorado (rather than CO)
Wyoming (rather than WY)

You may use D.C. for the District of Columbia in text, in both formal and informal publications. When you include addresses or state names in full addresses (containing streets and cities), contact lists, reference lists, and bibliographies, however, you may use the following abbreviations:

AL, AK, AZ, AR, CA, CO, CT, DC, DE, FL, GA, HI, ID, IL, IN, IA, KS, KY, LA, ME, MD, MA, MI, MN, MS, MO, MT, NE, NV, NH, NJ, NM, NY, NC, ND, OH, OK, OR, PA, RI, SC, SD, TN, TX, UT, VT, VA, WA, WV, WI, WY (PR, VI).

2. Countries

Do not abbreviate the names of countries (including the United States) when they are used as nouns. Use "U.S." as the adjective form.

the United States
U.S. Department of Energy program
U.S. population