When typing, some people use a single space after a sentence; others use two spaces. For the most part, I think these typing habits are meant to relate to two different typographic styles: double-spacing approximates what some call “english spacing”, while the single-spacing style is usually called “french spacing” (although “american spacing” might be more accurate for English-language text).
I freely admit that the choice between the two is merely a matter of style; neither is definitively “wrong”. As with other matters of style, however, the choices you make are interpreted as a reflection of your background and your values.
The history in brief
Single-spacing has always been the norm in French, but the double-spaced style was common for English-language typography until after the second World War. In the 1950s American publishers largely switched to french (single-)spacing, with the rest of the English-speaking world following soon after. I don’t think I’ve come across a professionally-typeset publication using “english spacing” that was printed in my lifetime.
Apparently the US government’s style guide recommended two spaces between sentences as late as 1959, but even this incredibly anachronistic guide (still dedicated mainly to typewriter typography) now recommends single-spacing:
2.49. A single justified word space will be used between sentences. This applies to all types of composition.
I was taught the “two spaces after a period” rule in an American high school typing class I took in the 1990s. I have little recollection of the exercise book we used, but it made little or no mention of computers and I would not be at all surprised if it was originally authored before 1960. I also dimly recall a recommendation from my mother that I use two spaces at the end of each sentence, based on a rule that she had learned in a typing class.
The TeX typesetting system, designed in the 1970s by Donald Knuth, has always added extra space between sentences by default, but this behavior can be changed with the
\frenchspacing command. Most of the common LaTeX styles I use (LNCS; Elsevier) automatically set this option.
Technical details of HTML make adding extra space between sentences much more cumbersome than simply hitting the space bar twice. Single-spaced sentences have always been the norm on the web.
How these styles are interpreted
Often what is interpreted as a stylistic choice is really nothing but a habit of which its owner is unaware. Still, lack of attention to one’s style sends a message of its own, as does the preference for habit over style. It would be difficult not to make assumptions about the social life of a man wearing mismatched clothes two decades out of fashion.
It’s tough to identify any strong impression I get from single-spaced sentences: single spacing is the norm in all professional typography, on the web, and in the vast majority of email (at least for those below the age of fifty).
Double-spaced sentences, however, are rare enough to draw my attention. I consider this novelty somewhat undesirable in its own right—style should not distract from content. Whether fair or not, I have quite a negative instinctive reaction to double spacing, for a number of reasons:
It betrays an ignorance of contemporary literature and design. A programmer unfamiliar with common coding conventions and idioms is a programmer who hasn’t worked with much pre-existing code; a writer unfamiliar with standard typography is a writer who doesn’t read.
It’s pretentious. Knowledge of the “two spaces after a period” rule is evidence of a formal education in typing; its application separates the writer from those without such an education.
It’s prescriptivist. “Two spaces after a period” is a classic example of a rule promulgated by authority which flies in the face of actual usage. Neither language nor style is subject to decree; both are natural phenomena which manifest and converge despite, not because of, attempts to codify them.
It’s old-fashioned. Styles change. Sixty years ago double-spaced sentences may have been common. They may come back into fashion in another sixty. In 2009, double spacing is an eccentricity.
Obviously these are just my own prejudices. I have friends who double-space their sentences, and I usually suggest that they reconsider their style in the same way I’d try to warn a friend about a particularly ugly shirt.
I’ll give the last word to Robert Bringhurst’s The Elements of Typographic Style:
2.1.4 Use a single word space between sentences
In the nineteenth century, which was a dark and inflationary age in typography and type design, many compositors were encouraged to stuff extra space between sentences. Generations of twentieth-century typists were then taught to do the same, by hitting the spacebar twice after every period. Your typing as well as your typesetting will benefit from unlearning this quaint Victorian habit. As a general rule, no more than a single space is required after a period, a colon or any other mark of punctuation. Larger spaces (e.g., en spaces) are themselves punctuation.