Indicate an abbreviation
Many abbreviations require a period. Dr, Mr, Mrs, and Ms do not take a period in British English, nor do most abbreviations taken from the first capital letters such as MA, Phd, or CIA. In American English, some of these do require periods or both usages are correct (with and without periods). If you require 100% accuracy in your punctuation, refer to a detailed style guide for the abbreviation usage rules in the variety of English you are using.