The “two spaces after period” rule was established during the days of typesetters, when additional space was needed to show the difference between the spacing between words and the spacing between sentences. There’s a common saying that using two spaces after a period is the surest sign the writer is older than 50. It is because when typewriters came around, they had only one font, and all the letters were monospaced, or took up the same amount of space. That means that the skinny “l” and wider “w” occupied the same amount of space on paper. People mimicked what they believed to be the format they’d grown used to by adopting two spaces after a period. That’s how the so-called two-space rule was born.
Do I Need One Space or Two After a Period?
With the dawn of computers, word processing programs not only began offering an irrational number of fonts, but they programmed each font to space characters proportionally. The “l” takes up about a third of the space a “w” does. In turn, most computer fonts will automatically give you enough room between sentences with one space.
There are, of course, exceptions. With academic work, spacing after a period can be mandated by your school’s chosen style guide. The Associated Press (AP Style) and the Chicago Manual of Style mandate a single space after a period. But The American Psychological Association (APA Style) dictates there should be two spaces after the period.
The rule of thumb is to use one space after a period unless explicitly told otherwise unless you are remaining true to tradition and using a typewriter.
Originally posted 6/30/2015 and happily updated 10/25/2017. Thanks for reading!