The Blue Book of Grammar and Punctuation: An Easy-to-Use Guide with Clear Rules, Real-World Examples, and Reproducible Quizzes, 11th Edition

Chapter 2

Punctuation

Periods

Rule 1. Use a period at the end of a complete sentence that is a statement.

Example:

I know him well.

Rule 2. If the last item in the sentence is an abbreviation that ends in a period, do not follow it with another period.

Incorrect:	This is Alice Smith, M.D..
Correct:	This is Alice Smith, M.D.
Correct:	Please shop, cook, etc. We will do the laundry.

Rule 3. Question marks and exclamation points replace and eliminate periods at the end of a sentence.

Commas

Commas and periods are the most frequently used punctuation marks. Commas customarily indicate a brief pause; they're not as final as periods.

Rule 1. Use commas to separate words and word groups in a simple series of three or more items.

Example:

My estate goes to my husband, son, daughter-in-law, and nephew.

Note: When the last comma in a series comes before and or or (after daughter-in-law in the above example), it is known as the Oxford comma. Most newspapers and magazines drop the Oxford comma in a simple series, apparently feeling it's unnecessary. However, omission of the Oxford comma can sometimes lead to misunderstandings.

Example:

We had coffee, cheese and crackers and grapes.

Adding a comma after crackers makes it clear that cheese and crackers represents one dish. In cases like this, clarity demands the Oxford comma.

We had coffee, cheese and crackers, and grapes.

Fiction and nonfiction books generally prefer the Oxford comma. Writers must decide Oxford or ...