Single Spacing is Un-American
Recently, my boss told me to stop using two spaces after a period. I’ve heard this before but felt that it was just a passing whim of language. Much like the saying “Cool beans,” which has thankfully passed.
Mr. Bossman explained that a computer is not a typewriter and that proportional fonts – as opposed to monospace typewritten fonts – have extra white space added to periods. It was an interesting point that led to the question: “Why did they double space on a typewriter?” 
Lemme tell ya now that the pitched battle between the Singlies and Doublies is far from over. In between jabs using thin spaces, em-dashes, and tildes, en-space mortar shots are ceaseless. The reason? We’ve been severely misled by dead typesetters.
There’s an excellent article in Wikipedia about “French Spacing” which is less a review of French punctuation and more a treatise on why we have been told to use only one space. The truth is pretty fascinating.
Before the Linotype revolutionized printing in 1885, standard typesetting added an em-space after a period. Replicating this on a Linotype was difficult due to a mechanical problem. Though workarounds were developed, Linotype operators preferred the same spacing after a period as between words. (For an interesting discussion that includes the personal experiences of a former Linotype operator, go to Typophile.)
What’s fascinating is that this one-space standard became encouraged and codified. Most likely, publishers realized they could save money by reducing the white space between sentences. Typesetters were satisfied because they prefer the look of full text without empty gaps. Because typesetters were the innovators of desktop publishing (DTP) these rules were enforced and the typewriter emerged as the scapegoat.
The Singlies argue that DTP applications use heuristics to determine the end of a sentence and add extra spacing. The logic and implementation of these heuristics is never explained. How could the program distinguish a proper noun following a punctuated abbreviation? Without a lexicon of all proper nouns it simply can’t. The truth is that the heuristics just don’t exist. There’s some added whitespace after a period. That’s it.
Take a look at the image below illustrating spacing using ems in InDesign:
The serif makes it hard to see, but the period and word spacing are differentiated by an extra point. One point. The blue line behind the period is one point wide.
Truthfully, the justification for increased spacing after a period in pre- and post-Linotype print is simple: It’s easier to read. Research has demonstrated that added space between sentences allows for improved reading and comprehension. Unfortunately, the legacy of the Linotype has begun to erode the quality of our printed text.
The image below illustrates the different spacing options available:
Notice that two spaces is almost exactly the same as adding an en-space.
As you can see in the em-space example, the font kerning causes the gap to be a bit larger than one em. 
At this point you may be wondering, “How is this un-American? I salute mom, god, and apple pies – am I actually a commie?” If you are one of the Singlies, then yes, you are a commie. Take a look at this:
That’s right, kids, it’s the Declaration of Indepence of the United Freakin’ States of America! The view above is the Dunlap Broadside which was printed on July 4th, 1776. Given the extreme importance of this document, it is safe to assume the Founding Fathers printed it using the highest standards of the day.
For us Doublies there remains some hope. An unsubstantiated blog comment indicates that some publishers prefer two spaces because it gives them greater control over the spacing without screwing up the author’s text. The debate on the aforementioned Wikipedia article continues and updates happen frequently.
Ultimately it is time to realize that the computer is neither a typewriter nor a Linotype.
Yr Fthfl Bddy,
Mike
For an overview of the challenges in adding visually distinct spacing to printed and online text, see my first comment entry.
Challenges to adding spacing
Since the article above got pretty long, I wanted to take the technical stuff out of it.
There are a couple things to consider about double-spacing with regard to automating text. John, my boss, brought up a valid point: Relying on authors to be consistent about spacing can be just as much of a problem as relying on heuristics.
The slightly longer truth about heuristics is that there are some out there, but they are not foolproof. TeX has an algorithm which, though good, fails to catch proper nouns following a period. The problem is illustrated below:
…the U.S.S. Franklin and the support of the U.S. Johnson and his associates…
If your style is to punctuate abbreviations, then it is impossible to determine if the above includes two sentences or one.
My friend Chris argues that the above illustration does not damn one-spacing, but that it demonstrates lazy writing. Though I fundamentally agree that the control of this problem rests with the author, I still would attempt to eliminate technical issues from the author’s palette. For instance, if we suddenly decided that the letter R is unattractive should we tell authors to stop using it? Pobably not.
My vote is for the author to double-space a submitted manuscript. This gives typesetters the freedom to strip the extra space and replace the normal space with something preferable. Even though mistakes can happen on the author’s end, I would rather keep content control out of the hands of typesetters.
For online work, I highly recommend double-spacing as well. Even though HTML disregards added space, there are plenty of reasons to keep it in. Speach synthesizers for the blind add a pause for each space in a text. With only one space after a period there is not enough of a pause to aid comprehension. Additionally, some less visually impaired people will read all websites in plain text using a typewriter font.
Similarly, e-mail can be read as either rich text or plain text. Whether you are visually impaired or not, the lack of a second space makes comprehension difficult.
I cannot find a good argument for retaining the one-space style in manuscripts or electronic work.
The typesetting used for the above article above (with the respective & signs):
Em-spaces after sentences (#8195;)
Thin spaces in addition to a word space after commas (#8201;)
Thin space, en-dash, thin space for em-dashes (#8201;, #8211;, #8201;)
Non-breaking spaces before titles (nbsp;)
I wanted to do typeset this article correctly. Although it wasn’t much effort, I’ll probably just stick to double-spacing.
I’m sorry. I couldn’t read this. All the white space blinded me.