General Rules
* Traditional Style: Periods are used after most abbreviations, especially for titles like Mr., Mrs., Dr., etc.
* Modern Style: Periods are often omitted in abbreviations that are commonly understood without them, such as USA, NATO, and etc.
Examples
* Traditional: Mr. Jones, Dr. Smith, U.S.A.
* Modern: USA, NATO, etc.
Specific Cases:
* Units of Measurement: Periods are generally omitted (kg, km, cm).
* Initials: Periods are used when initials stand for separate words (J.R.R. Tolkien).
* Latin Abbreviations: Periods are often used with Latin abbreviations like i.e., e.g., and etc.
Style Guide:
Always refer to the style guide for your specific field or publication. For example:
* AP Style: Generally avoids periods after abbreviations.
* Chicago Manual of Style: Follows traditional rules with more flexibility for modern abbreviations.
In Conclusion:
The use of periods after abbreviations can vary depending on the specific case and the style guide you are using. It's always best to check the specific rules for your context.