Can I let you in on a secret? Typing two spaces after a period is totally, completely, utterly, and inarguably wrong. And yet people who use two spaces are everywhere, their ugly error crossing every social boundary of class, education, and taste. * You'd expect, for instance, that anyone savvy enough to read Slate would know the proper rules of typing, but you'd be wrong; every third e-mail I get from readers includes the two-space error/Farhad Manjoo, Slate. More here.
Question: Do you use one or two spaces after sentence-ending periods?