If you learned to type in a formal class (pre-word processing days, so long ago), you learned to type two spaces after a period. That was standard practice in the days of the typewriter that featured monospaced fonts. That is, every letter took up the same amount of space on the line whether it was a “w” or an “i.” Two spaces after a period enhanced readability.
(Sidebar: Maybe you remember when IBM’s “Selectric” typewriter was all the rage, featuring the type ball rather than the basket of keys. And we could actually change typeface 
by changing the ball. I recall two faces [Prestige and Elite] and two sizes [10 and 12 pitch]. And wow, there was actually the numeral 1 included, so we no longer had to use a lower case letter “l” as a substitute. We were livin’ large then.)
Enter word processing and every conceivable font. Fonts that offer proportionate spacing. The skinny “i” takes less space on the line than the “w.” As it should. Proportionate spacing negates the need for two spaces after the period or any punctuation, for that matter. It’s a hard habit to break for a lot of people. But it’s true: one space only. Read any newspaper, book, or magazine and you’ll see only a single space between sentences. It’s a long-standing typography practice.
Not convinced? Read this paragraph again with two spaces used:
Enter word processing and every conceivable font. Fonts that offer proportionate spacing. The skinny “i” takes less space on the line than the “w.” As it should. Proportionate spacing negates the need for two spaces after the period or any punctuation, for that matter. It’s a hard habit to break for a lot of people. But it’s true: one space only. Read any newspaper, book, or magazine and you’ll see only a single space between sentences. It’s a long-standing typography practice.
It probably looks a bit odd, and quite possibly, you found yourself stalling ever so slightly at the end of each sentence.
If you can’t seem to break the habit, you can always easily fix it after the fact. Use the find-and-replace feature in your word processing program. Replace two spaces with a single one, and continue to do so until the result returns “zero replacements made.”
