Abbreviations and acronyms may be used on first reference only if widely recognized.
- DNA
 - FBI
 - NASA
 
Otherwise, spell out the complete name or phrase on first reference and follow with the abbreviation in parentheses if and only if the abbreviation will be referenced subsequently.
- The collaboration includes Brown University and Massachusetts General Hospital (MGH).
 
In general, abbreviations and acronyms with only two capitalized letters use periods; for those with three or more capitalized letters, omit the periods between the letters.
- U.S.
 - CIA
 
When an acronym serves as a proper name and exceeds four letters, capitalize only the first letter.
- Unesco
 - Unicef
 
When an abbreviation follows an indefinite article, choose between “a” or “an” as determined by the way the abbreviation would be read aloud.
- an HMO
 - a UFO
 
(See: “academic degrees”)
(See: “state names”)