As you likely know, the period is the punctuation mark used to end declarative sentences. Unless you are writing a question or an exclamation, chances are you need a period at the end. But how many spaces come after that period? It seems that question has been answered: One. Just one.
According to every major style guide you’ll find, the rule is a single space after a period or any other punctuation mark you use to end a sentence. Even the APA, the staunchest defender of the double space over the decades, changed their stance on the issue in 2019. In 2020, Microsoft also struck a major blow to all the double-spacers out there when it officially categorized a double space after a period as a writing mistake in their popular Microsoft Word program.
That being said, academic style guides like APA and MLA allow for some leeway. If an instructor tells a student to use two spaces after periods, these style guides advise a student to follow the instructor’s preference. No reason to risk a teacher’s wrath over punctuation squabbles, right?
Evolution of two spaces after a period
According to research, the preference for two spaces after a period falls along generational lines. Studies have shown that, beginning with millennials, younger generations widely prefer the single space after a period. On the flip side, Gen X and older generations usually prefer the double space. So, what happened?
To answer that question we need to go pretty far back in time—centuries back in time, in fact. Although typewriters are often blamed as the origin of the “double space after a period” rule, this preference actually predates typewriters by at least … 200 years. (As you can clearly see in this Bible from the 1600s.) The double space (or even larger spaces) was the norm in English books for a really long time.
This preference started to weaken a bit in the 20th century when it became cheaper and easier to mass publish books. However, the wide use of those tricky typewriters kept the momentum behind the two space preferences. In the late 20th century, though, everything would change.
Personal computers would become commonplace, and computer keyboards would put an end to the wide use of typewriters. Without having to worry about issues with typesetting, the single space would quickly become the norm as single spacing saved writers some time and meant more text could fit on a page.
This explains the generational gap; younger generations have used computers their whole lives and have never had to worry about using a typewriter. Because keyboards are now the standard for the foreseeable future, it seems likely that the double space will eventually become a thing of the past.
When is it acceptable to use two spaces after a period?
The single space is, with few exceptions, king in formal writing. For this reason, you are most likely going to see all media outlets use only single spaces and you’ll be expected to do so as well if you want to write for them. Based on the advice of the style guides, the only time it is acceptable to use two spaces after a period in formal writing is if an instructor told you to.
Informally, though, the choice is still yours. If you’re writing out a holiday card or posting hilarious cat memes on Twitter, you can do what you like. Just be aware that the single space has a lot more support and is now widely considered to be the “correct” choice.
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