As with so many aspects of the writing world, formatting issues are a gray area, and writers struggle with the burning question: One space or two after closing punctuation?
When in doubt, creative writers often turn to the Chicago Manual of Style, whose official view is that there is no good reason to use two spaces after a period for work that is to be published. (Obviously, this rule does not apply to personal correspondence, notes, etc.)
In the days of typewriters, extra space was necessary to create a more defined space between sentences for the reader's eye. Typewriter fonts are monospaced, which means that all the letters take up the same amount of space, and most of us were taught in typing class to add that extra keystroke at the end of a sentence. But computerized fonts are proportionally spaced, and a single space is sufficient to provide a visible break. The exceptions are the fonts Courier and Monaco, which are monospaced, but it's probably best to switch to a font such as Times New Roman or Arial rather than using the double space.
So save yourself a keystroke; there's something to be said for efficiency.
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