Rules for writers, Tenth edition - Diana Hacker, Nancy Sommers 2021
The period
End punctuation
Punctuation
Use a period to end all sentences except direct questions or genuine exclamations. Also use periods in abbreviations according to convention.
To end sentences
Most sentences should end with a period. A sentence that reports a question instead of asking it directly (an indirect question) should end with a period, not a question mark.
In abbreviations
A period is conventionally used in the following abbreviations.
✵ Mr. |
✵ i.e. |
✵ a.m. (or AM) |
✵ Ms. |
✵ e.g. |
✵ p.m. (or PM) |
✵ Dr. |
✵ etc. |
NOTE: If a sentence ends with a period marking an abbreviation, do not add a second period.
Do not use a period in abbreviations of organization names, academic degrees, states, and designations for eras.
✵ NATO |
✵ UCLA |
✵ BS |
✵ TX |
✵ BC |
✵ IRS |
✵ NIH |
✵ PhD |
✵ NY |
✵ BCE |