The first rule a child learns about the period (Br. “full stop”) is that it ends a sentence.
Where else is this little dot used, and when is it omitted?
Abbreviations may be written with or without periods.
Whether or not to use a period with the abbreviations for honorifics, such as mister, miss, doctor, etc., depends upon regional or professional usage.
British style omits the period when the abbreviation ends in the same letter as the word being abbreviated.
Miss or Mrs=Ms
US style calls for a period after these same abbreviations:
In proper names, initials are followed by a period:
Harry S. Truman
NOTE: There’s a story that, because the S in Harry Truman’s name does not stand for a particular name, it does not take a period. In practice, the S in Harry S. Truman does take a period.
For the few famous people known by their initials only, no periods are required:
Franklin Delano Roosevelt=FDR
John Fitzgerald Kennedy=JFK
US state abbreviations
In addition to their two-letter postal codes, which do not use periods, US states have abbreviations that use periods. The Chicago Manual of Style prefers the postal codes in most contexts, together with US as the abbreviation for United States.
The AP Stylebook, on the other hand, recommends using the postal codes only in the context of a complete address. Otherwise—with eight exceptions—the traditional abbreviations are used in datelines and text, and United States is abbreviated U.S.
NOTE: When a sentence ends with an abbreviation, the period that belongs to the abbreviation also serves to end the sentence.
The eight states never abbreviated by AP are Alaska, Hawaii, Idaho, Iowa, Maine, Ohio, Texas, and Utah. District of Columbia is also spelled out in datelines and text.
Acronyms and Initialisms
The period is not used with acronyms or initialisms. Both acronyms and initialisms are written without spaces.
They may be written in all capitals, lowercase, or a combination thereof. Although they never contain periods, they may contain other symbols, such as a hyphen or an ampersand.
A word formed from the initial letters of other words or (occasionally) from the initial parts of syllables taken from other words, the whole being pronounced as a single word.
HAZ-MAT Hazardous Material
LASER—Light Amplification by Stimulated Emission of Radiation
NaNoWriMo—National Novel Writing Month
NASA—National Aeronautics and Space Administration
RADAR—Radio detection and ranging
SCUBA—Self-Contained Underwater Breathing Apparatus
SWAT—Special Weapons and Tactics
A group of initial letters used as an abbreviation for a name or expression, each letter or part being pronounced separately.
FTP File Transfer Protocol
FYI For Your Information
ESPN Entertainment and Sports Programming Network
RN Registered Nurse
USB Universal Serial Bus
NOTE: The terms were once used interchangeably, but modern usage distinguishes acronyms from initialisms. The general distinction is that initialisms are pronounced as individual letters, whereas acronyms are pronounced as words. Nevertheless, some shortened forms combine both initial letters and pronounceable elements. For example:
B&B—Bed and Breakfast
CD-ROM—Compact Disk-Read Only Memory
C-SPAN Cable-Satellite Public Affairs Network
No punctuation is needed when a photo caption is an incomplete sentence. A caption that is a complete sentence does require a period.