Double or Single Space?

Normally, I’m not one to complain of grammatical errors. You can sometimes find me writing run-on sentences, forgetting commas and creating sentence fragments. Hey, I’m no copywriter. But as a designer, there is one grammatical error that gets me boiling: double spacing after a period.
Now, I know there are many out there that say the double space after the period is the right thing to do. Just because your middle school English teacher says it was correct, doesn’t mean it’s actually the proper way. It’s grammatically illegal, folks (and in my opinion, should be punished by full force of the grammar law).
So, if it’s not correct to double-space, why did we begin to do it? In the 20th century typesetters all over the world came together and decided we should use one space after the period. This is a shared belief among all typesetters. Then, the manual typewriter was invented. Cue doomsday music. The manual typewriter used monospace typing, meaning that every character occupies an equal amount of horizontal space. Skinny characters like “i” and “t” are given the same space as fat letters like “o” and “m” creating uneven, white spaces between letters and words. The two-space rule came into play to make the text easier to read and making it more obvious the sentence had ended.
Today, almost every font on your PC or Mac has proportional typesetting – less horizontal space is given to skinny characters and more space is given to fat ones. This is why we don’t need the double-space after the period. I repeat, we DON’T NEED two spaces after a period. Most of us aren’t’ using typewriters, however, we are still typing like we do. So, unless you are using a typewriter, please refrain from giving the designer next to you a heart attack and use ONE space.