1. Periods:
* Standard abbreviations: Most abbreviations ending with a period are followed by a period.
* Examples: Dr., Mr., Mrs., Ms., Inc., U.S., a.m., p.m., etc.
* Exceptions:
* Abbreviations that are pronounceable as words like NATO, laser, and radar don't usually use periods.
* Some abbreviations used in technical fields, like "rpm" (revolutions per minute), may omit the period.
2. No Punctuation:
* Acronyms: Acronyms are pronounceable abbreviations formed from the first letters of words, like NASA, UNICEF, and UNESCO. They don't need periods.
* Proper nouns: Some proper nouns, like "IBM" or "CIA," are abbreviations, but they are treated as regular words and don't require punctuation.
3. Other Punctuation:
* Sentence-ending abbreviations: If an abbreviation ends a sentence, it's followed by a period.
* Internal punctuation: If an abbreviation is part of a larger sentence, it should follow the usual punctuation rules. For example, "The company is located in the U.S."
In summary:
* Standard abbreviations: Use a period after the abbreviation.
* Acronyms and pronounceable abbreviations: Don't use periods.
* Proper nouns: Treat them as regular words and follow the usual punctuation rules.
* Sentence-ending abbreviations: Use a period after the abbreviation.
If you're unsure about the punctuation for a specific abbreviation, consult a style guide or a dictionary.