Names of Countries
1. Spell out in text and abbreviate in tables, lists, and the like.
United States (use rules 2 and 3 for United Kingdom as well)
1. Use periods with the abbreviation except in scientific text.
2. Use the abbreviation when used as an adjective, e.g., U.S. currency or U.S. involvement in international politics.
3. Spell out when used as a noun, e.g., the United States was established in 1776, but it was not recognized as such until 1783.