How many spaces after a full stop?

A colleague sent around the following email, which prompted some thoughts about writing style that segued into the hot topic du jour:

I was taught as a young dude that there should be *two* spaces following a period that concludes a sentence (prior to beginning a new sentence in the same paragraph). For some reason, the advent of word processors (and perhaps a mania for saving space on the page) has led some people to advocate the reduction of this number to *one* space. I find this unsightly (if not unseemly) and cannot bring myself to break the habit of following the period with two spaces. Is there a consensus on which is the proper style?

The Chicago Manual of Style's website relates:

The view at CMS is that there is no reason for two spaces after a period in published work. Some people, however—my colleagues included—prefer it, relegating this preference to their personal correspondence and notes. I’ve noticed in old American books printed in the few decades before and after the turn of the last century (ca. 1870–1930 at least) that there seemed to be a trend in publishing to use extra space (sometimes quite a bit of it) after periods. And many people were taught to use that extra space in typing class (I was). But introducing two spaces after the period causes problems: (1) it is inefficient, requiring an extra keystroke for every sentence; (2) even if a program is set to automatically put an extra space after a period, such automation is never foolproof; (3) there is no proof that an extra space actually improves readability—as your comment suggests, it’s probably just a matter of familiarity (Who knows? perhaps it’s actually more efficient to read with less regard for sentences as individual units of thought—many centuries ago, for example in ancient Greece, there were no spaces even between words, and no punctuation); (4) two spaces are harder to control for than one in electronic documents (I find that the earmark of a document that imposes a two-space rule is a smattering of instances of both three spaces and one space after a period, and two spaces in the middle of sentences); and (5) two spaces can cause problems with line breaks in certain programs.

As both my colleague's email and the CMS commentary suggests, there was a convention back in the manual typewriter days of using two spaces after a full stop, but the advent of wordprocessing seems to have undermined, if not wholly eliminated, that convention. (I note, however, that the rise of hte internet and concomitant HTML style conventions may have been an even more important factor.) John Rhodes reports:

In late September 1998 I asked several people what I thought was a simple typographic question. I was thinking about doing a quick eye tracking study with a colleague and I wondered if there was any empirical research available that showed that a single space after a period was "better" than two spaces, or vice versa.

While this was originally a question about web usability, I realized that the spacing issue is basically irrelevant! Steve Outing of Planetary News correctly pointed out that almost all browsers will only render one space after the period, regardless of how many spaces you put into the HTML code. Note however, that spaces can be hard-coded into the HTML using " ". ...

One of the next things I realized is that, in general, the spacing after a period will be irrelevant since most fonts used today are proportional. That is, each character is not the same size. Amy Gahran of Contentious kindly indicated that very few monospaced (i.e., non-proportional) fonts are used today on the Internet, except for perhaps Courier. This is generally the case offline as well. Proportional fonts are used much more often. For your information, I did not get any good feedback about the various fonts that are used for other Internet applications and activities, such as chat rooms and email. My guess is that they are mostly monospaced, but it is just a guess.

Like Amy Gahran, Kathy Gill told me that the current typographic standard for a single space after the period is a reflection of the power of proportionally spaced fonts.

"The only reason that two spaces were used after a period during the 'typewriter' age was because original typewriters had monospaced fonts -- the extra space was needed for the eye to pick up on the beginning of a new sentence. That need is negated w/proportional space type, hence [it is] the typographic standard."

While I'm on the topic, here's what one of the folks at The Design & Publishing Center had to say about the history of typographic spacing: "In the days of typewriter manuscripts the extra space was necessary to separate the ends and beginnings of sentences. The space character never changed. With the advent of electronic typesetting, the software attempts to 'fit' the type to specific line lengths, it both expands or contracts the available space to make the type fit. Word spacing is where most of this space 'play' takes place."

The relevance to Ra^thergate is self-evident. In all the reporting on the Killian memos, however, I have yet to see any reportage on the number of spaces following the full stops. Has anybody tried to figure out whether there are one or two? If so, email me and I'll post a link. Update: Steve Minor says there are two spaces!