Periods: We Go Way Back!
The period seems pretty straightforward. After all, for most of us, this was the first punctuation we learned when we were learning to read. See Jane run. To our first-grader selves, that meant, See Jane run, stop, take a break, keep going. You can do this thing.
Periods are certainly important punctuation because they are what we most often use to separate complete thoughts or independent clauses. Periods are how we end our sentences most of the time.
However, periods do serve another important function in the world of punctuation: You should use a period with abbreviations.
Mr.
Mrs.
Ms.
a.m.
p.m.
Then, of course, there is the question about how many spaces should come after a period. If you learned to type on those things called typewriters, you learned to double space after all periods at the ends of sentences. However, this isn’t always the case anymore. Generally, we just single space after all periods, though you should consult the style guide you are using. APA style now requires a single space after a period at the end of a sentence, after requiring a double space in the sixth edition.
The lesson here is clear. The “rules” change all the time and are dependent upon your style guide. It’s important to stay up to date.