"How many spaces after a period?" I hear this question all the time. It's a big debate that can go on for hours for one simple reason: There's an answer, but it's not set in stone. Let me explain.
ALL THE WAY BACK TO GUTENBERG
The story goes all the way back to Gutenberg and the invention of movable type. At that time, there were no rules about this and printers used variable spacing after a period. That is, they put in the amount of space they thought was necessary for the reader to know a sentence ended. Gradually English printers came to use an ‘em’ space between sentences, which is approximately what we would call two spaces after a period. (The French, always proud of any language differences, continued to use only one space.)
Printing moved along for several centuries not changing much as least regarding this question. Then we came to the 19th century and the invention of typewriters. Suddenly, offices could produce documents that looked as if they were printed, but were actually done in-house. In that way, typewriters revolutionized the printing business. But there was a catch. Typewriters used “monospaced type.” Every letter or character took up the same amount of space on the key. Inconveniently, some letters such as "m" and "w" were wider when typed in a sentence. They took up more space. So typists began to insert two spaces after the period for clarity. That's where the old "two spaces" after a sentence rule originated.
THE CONVENTION TODAY IS ONE SPACE
Now jump to the 20th century. Mechanical typesetting and computers began to take over. Suddenly, letters took up only as much space as they had to. Mechanical typesetting was very different from typewriter typing. So a single space was wide enough to show the reader that the sentence had ended. From the mid 20th-century onward, typographers and style experts fell in line with this “one space” convention. Double spacing faded away. Single spacing is the standard today.
But wait. Diehards in certain fields still insist on double spacing. That's why the question still causes controversy. Certain academics and even some publishing houses (so I've heard, although no one gives precise names) still insist on two spaces. But in the business world in general, one space prevails. So for now, you can stick to that, and, if anyone asks why, you can put on your "grammar guru" hat and explain it to them.