I know you have all lost many hours of sleep over this question, so I’m here to relieve your stress. For a journal editor, few tasks are more tedious than going through a document and fixing all the incorrect spacing at the ends of sentences, so please, if you ever decide to send me anything, read this article first. The correct number of spaces after a period is one! Always!
Why were you taught to put two spaces after a period when you were a kid? As this author explains, it’s the fault of the manual typewriters in use during the mid-20th century, which made it difficult to distinguish a single space at the end of sentence. So typesetters made a concession to the single-space rule to accommodate their use.
The readability problem began to go away in the 1970s when the typewriter manufacturers started shifting to proportional typesetting, and today it’s completely gone (unless you use the Courier font . . . why would you do that?). But, like a zombie, the two-space idea continues to hang around, as evidenced by half or more of the article and book review submissions that hit my inbox.
So please help to end the two-space tyranny! Send the typing teacher at your local schools this article!