Rutgers University
Rutgers University: Guide to Grammar and Style: Periods
An excellent outline of the usage and rules concerning the "period" in English punctuation. Includes guidelines about acronyms, abbreviations, sentence ending, and more.
Other
Sandhills Community College: The Period
A good guide to understanding the several uses of the period in English punctuation. Includes information concerning periods in a sentence as well as abbreviations. Includes several examples.
ACT360 Media
Writing Tips: Punctuation: Period
A brief description of the rules, usage, and examples of the "period" in English punctuation. Includes basic information concerning period use in abbreviations as well.