Typographical rules are sometimes based on empirically proven ease of reading, but much more often they are simply tradition or aesthetic preference. There’s nothing inherently wrong with that, but it gets annoying when such accidental habits are declared absolute truths or, worse, justified with made-up history. And that is the case with the often-heard claim that “two spaces after a sentence are bad and a relic of the typewriter era.”
This myth is thoroughly destroyed by three 2011 articles I recently came across. Mark Barrett’s Two Spaces After a Period argues from aesthetics and experience, providing a number of screenshots to illustrate how the extra spacing can be sometimes helpful. Later in the year, an anonymous author writing as “Heraclitus” presents mountains of historical evidence in two articles, Why two spaces after a period isn’t wrong (or, the lies typographers tell about history) and The Chicago Manual of Style and a single space after periods. His findings are summed up in the following quotes:
There were earlier standards before the single-space standard, and they involved much wider spaces after sentences.
Typewriter practice actually imitated the larger spaces of the time when typewriters first came to be used. They adopted the practice of proportional fonts into monospace fonts, rather than the other way around.
Literally centuries of typesetters and printers believed that a wider space was necessary after a period, particularly in the English-speaking world. It was the standard since at least the time that William Caslon created the first English typeface in the early 1700s (and part of a tradition that went back further), and it was not seriously questioned among English or American typesetters until the 1920s or so.
The “standard” of one space is maybe 60 years old at the most, with some publishers retaining wider spaces as a house style well into the 1950s and even a few in the 1960s.
As for the “ugly” white space, the holes after the sentence were said to make it easier to parse sentences. Earlier printers had advice to deal with the situations where the holes became too numerous or looked bad.
The primary reasons for the move to a single uniform space had little to do with a consensus among expert typographers concerning aesthetics. Instead, the move was driven by publishers who wanted cheaper publications, decreasing expertise in the typesetting profession, and new technology that made it difficult (and sometimes impossible) to conform to the earlier wide-spaced standards.
For my own part, I vacillate between one and two spaces but generally use a single space with good proportional typefaces (such as Georgia on this blog). Ultimately, though, I prefer the slightly expanded spacing of the TeX typesetting system. I think it strikes an optimal balance between a visual separation of sentences, and maintaining a pleasant appearance of the entire text block. That is also the conclusion of “Heraclitus:”
While the modern convention is the single space, it is no less arbitrary than any other, and if you believe that larger spaces after periods look better in some situation, you should feel confident that your choice is supported by hundreds of years of good typographical practice. For the record, […] my preference is not for double-spacing, but for a slightly larger sentence space, about 1.5 times an interword space for most typefaces. But unlike the modern single-space fanatics, I don’t judge anyone’s aesthetic preferences, nor will I try to make up fairy tales using fabricated history to convince you.
2012-12-18: Thomas A. Fine provides some great resources on the subject. He wrote an extensive overview article, built a nice demonstration of variable Sentence Spacing in HTML and CSS, and documents his ongoing research at Sentence Spacing.