People, we should be using only a single space between sentences in electronic documents. Many of us still use two!
On the topic of this practice, Slate says,
Can I let you in on a secret? Typing two spaces after a period is totally, completely, utterly, and inarguably wrong.
And the Chicago Manual of Style agrees:
But introducing two spaces after the period causes problems: (1) it is inefficient, requiring an extra keystroke for every sentence; (2) even if a program is set to automatically put an extra space after a period, such automation is never foolproof; (3) there is no proof that an extra space actually improves readability—as your comment suggests, it’s probably just a matter of familiarity (Who knows? perhaps it’s actually more efficient to read with less regard for sentences as individual units of thought—many centuries ago, for example in ancient Greece, there were no spaces even between words, and no punctuation); (4) two spaces are harder to control for than one in electronic documents (I find that the earmark of a document that imposes a two-space rule is a smattering of instances of both three spaces and one space after a period, and two spaces in the middle of sentences); and (5) two spaces can cause problems with line breaks in certain programs.
So, in our efficient, modern world, I think there is no room for two spaces after a period. In the opinion of this particular copyeditor, this is a good thing.
Here’s where the two-space spacing came from and the explanation of why today, one space is enough.
In the nineteenth century, writers were encouraged to add one space between sentences. Twentieth century typists were taught to press the spacebar twice at the end of every sentence. In modern typesetting, only one space should be left between sentences because that is how typographers designed the typefaces in order to enhance the beauty of the text. Adding extra spaces ruins the harmonic flow of the text. Thankfully, all white space in html, in any combination of spaces, is automatically collapsed into a single word space. One space between sentences, or French spacing, is the current convention in countries that use the modern Latin alphabet; two spaces between sentences, or English spacing, is the historical American typing convention which is no longer used in modern media.
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Updated on 27 February 2012 to include more references.