Putting periods after abbreviations (or not) is a little-known difference between British English and American English. Here's when you need periods and when you don't.
A reader named Dennis K. asked whether you always put periods after abbreviations or whether it’s different depending on which letters from the word are used in the abbreviation. Someone told him that abbreviations should only use a period if it doesn’t end with the last letter of the original word, but he’d never heard that before, so he was wondering if it is right.
Putting periods after abbreviations (or not) is a little-known difference between British English and American English.
In American English, we always put a period after an abbreviation; it doesn’t matter whether the abbreviation is the first two letters of the word (as in Dr. for Drive) or the first and last letter (as in Dr. for Doctor).
British writers, however, make a distinction: abbreviations that are written with the first and last letter of the word (as in Dr. for Doctor and Mr. for Mister) do not get a period.
Here are examples of how the abbreviations differ:
“Personal Names with ‘Saint’ or ‘St.’” The Chicago Manual of Style, 16th edition. 10.27 http://www.chicagomanualofstyle.org/16/ch10/ch10_sec027.html (subscription required, accessed September 17, 2014)
“Punctuation in Abbreviations.” Oxford Dictionaries. http://www.oxforddictionaries.com/us/words/punctuation-in-abbreviations-american (accessed September 17, 2014)