This debate marches on, even today when updated information is so readily available. Yes, your high school typing teacher was misguided: two spaces after a period is not really a good practice. I will stop short of saying it’s “wrong” because ultimately, it’s a style decision. If you really like the way two spaces looks, have at it! But I am likely to disagree with you. [grin]
There’s also some discussion over why two spaces after a period became a common practice to begin with. Many believe it is a carryover from the days of monospaced typewriters, and that is a logical assumption. On a typewriter, users needed something to make the start of a sentence stand out a little more and two spaces accomplished it. Nowadays, with proportional fonts, the font itself has kerning tables which specify how much space looks best between letters, digits and punctuation. This spacing was designed and implemented by the font’s designer.
That leads us to the real origin of double spaces. Back before typewriters, when fonts were actually little chunks of metal, an em space (blank space the equivalent width of the letter M) was often used after a period. This created a visible break in the paragraph, making the start of a sentence stand out. When typewriters appeared, after 1867, users needed a way to recreate that same typographical look they had grown accustomed to in printing. On a typewriter, every character lines up vertically on a grid (monospace), so the only choice was to insert two spaces after a period. And that convention stuck. So hard in fact, it is still taught in some typing classes today.
On a related note, if you are doing HTML for the Web, browsers do not recognize more than one consecutive space. So, even if you insert them, they won’t be visible on your website. You can work around this by inserting the ASCII character for an additional space but that’s an awful lot of additional effort for little return.
If you’d like some more information about style practices regarding single or double spaces after a period, here are some great resources:
Chicago Manual of Style
“6.11 Space between sentences. In typeset matter, one space, not two (in other words, a regular word space), follows any mark of punctuation that ends a sentence, whether a period, a colon, a question mark, an exclamation point, or closing quotation marks.”
Associated Press Stylebook
“Use a single space after a period at the end of a sentence.”
So, let me know what your preference is. Do you use single or double spaces after a period and why? Have you changed this practice at some point?