I, too, was taught to type two spaces after a period. That was in the late fifties, as a matter of fact. As soon as I learned to type, I hardly ever used handwriting again. I typed, using two spaces after every period, through high school, through college, through graduate school. I typed two spaces after every period on punch cards, on paper tape, in FØRTRAN comments, in SNOBOL comments, in C comments, in every computer context that wasn’t going to be parsed by machine. I typed two spaces after every period in TECO, in RUNOFF, in Word-11, in AppleWriter, in WordStar. It was a fixed habit that I probably practiced an average of several hundred times a day, every day, for over thirty years.

Then I got my first Macintosh, and discovered that typing two spaces after the period is not appropriate in proportionally-spaced type. (Which I should have known anyway, because I belonged to the high-school printing club and learned how to set type by hand in a composing stick).

At about the same time, I learned to use italics for emphasis instead of underlining, and that an open quote is different from a close quote.

Usages do change with time, and while I am a crotchety middle-aged guy who is set in his ways and has the illusion that he is Upholding Standards, I try not to be too hidebound about it.

And I have stopped typing two spaces after every period.

Because… it is incorrect.