period

this from the atlantic monthly's word court, by barbara wallraff:

ADAM GORDON, of Los Gatos, Calif., writes, “At the advertising and marketing agency where I work, we have an ongoing debate about the number of spaces between a terminating period and the first letter of a new sentence. We writers were all taught to use two; my artists insist that one is the current rule. Would you be so kind as to adjudicate?”

Do anything you like in letters, e-mail, business memos, and other writing that’s an end in itself, but put one space between sentences in writing that’s going to be published, whether in print or on the Web. It’s standard.

This wasn’t always the case. You can find extra space between sentences in books from as late as the 1960s. No doubt this was in part an aesthetic choice. But I suspect that extra space between sentences became common mainly because type used to be set by hand, and it was easier to justify the lines by adding substantial spaces between sentences than by inserting smaller spaces between all the words. When automated typesetting came in, the machines could readily add space throughout the line, and the old practice died away. Then came computerized typesetting. In its early days, two spaces between sentences that came at the end of a line sometimes turned into one awkward space at the beginning of the next line—so publishers told typesetters to break the two-space habit if they hadn’t already. And here we are.

and now we know.