Think McFly, think!
I had a fun debate with an ex about this one. I’ve always used a single space between sentences. In the business world, these corporate robots are constantly placing two, three, even FOUR spaces between their sentences.
The ex was adamant that you had to use two spaces between sentences. Of course, she works in PR and is consistently exposed to a bunch of chuckleheads masquerading as writing professionals. I kid. Sort of.
One space will do. Why are you forming such a wide berth? You have enough words on the page. You don’t need to leave room for later additions. A period denotes the end of one sentence and the start of the next. A little extra insurance really isn’t necessary.
So why do so many people do it? Ah, thanks for asking. It’s a pretty interesting anecdote.
An article in Slate puts this argument to rest like a boss. According to Slate writer Farhad Manjoo,
The problem with typewriters was that they used monospaced type—that is, every character occupied an equal amount of horizontal space. This bucked a long tradition of proportional typesetting, in which skinny characters (like I or 1) were given less space than fat ones (like W or M). Monospaced type gives you text that looks “loose” and uneven; there’s a lot of white space between characters and words, so it’s more difficult to spot the spaces between sentences immediately. Hence the adoption of the two-space rule—on a typewriter, an extra space after a sentence makes text easier to read.
To quote the great old balls Biff Tannen, “You sound like a damn fool when you say it wrong!”