Appear more youthful instantly — and without the need for any painful filler injections. This tip is simple and doesn’t require a trip to the dermatologist: Simply stop using two spaces after sentences. The double space is a telltale giveaway that you spent your youth hunting and pecking away on a Smith-Corona rather than a laptop. Give your content a youthful glow — and earn the undying gratitude of proofers everywhere — by remembering to insert just one space after periods.
Students of typography know the reason that double spacing after end marks became standard. Early typewriters used only fixed-width (otherwise known as monospaced) fonts. So using double spaces after periods made it easier to differentiate the start of a new sentence in text where an “I” took up the same amount of space as a “W” or an “M.”
But the arrival word processors and computers changed all that. Along with the new technology came the popularization of variable-width fonts, where skinny letters take up less space than chunkier ones. To this day, variable-width fonts are by far the most common. However, some monospaced fonts remain, including Courier, which was designed in the 1950s and kind of looks like it.
So bring your writing into the modern era by making this one simple change. And in case you were wondering, single spacing after periods is indeed correct formatting according to the Associated Press Stylebook, The Chicago Manual of Style, and the Publication Manual of the American Psychological Association.
Source