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The full stop (Commonwealth English), period (North American English), or full point (.), is a punctuation mark used for several purposes, most often to mark the end of a declarative sentence (as distinguished from a question or exclamation).
A full stop is frequently used at the end of word abbreviations—in British usage, primarily truncations such as Rev., but not after contractions which retain the final letter such as Revd; in American English, it is used in both cases. It may be placed after an initial letter used to abbreviate a word. It is often placed after each individual letter in initialisms, (e.g., U.S.), but not usually in those that are acronyms (NATO). However, the use of full stops after letters in initialisms is declining, and many of these without punctuation have become accepted norms (e.g., UK and NATO).