Most common abbreviations end in a period: Mr., Dr., a. m., etc., Tues., Sept . Other abbreviations are written without a period. In general, you can omit periods for abbreviations written in capital letters ( FBI, CIA, IOU, CNN) if the abbreviation doesn't spell out another word. For example, USA is acceptable, but M.A. should include periods since it could be mistaken for a capitalization of the slang word ma. Most abbreviations ending in a lowercase letter should still be written with periods: yr., mo. Exceptions are mph, rpm, and metric measurement abbreviations such as ml, cm, and gm. Do not use periods with state abbreviations: AZ, CA, NY, WY, and so on. If you are uncertain about using periods with an abbreviation, check a dictionary or style guide.
Period with quotation marks
Always place the period inside quotation marks, whether or not it is part of the quotation.