One Space or Two?

Q. Please help. I have confusion regarding the correct spacing after periods and other closing punctuation. My company uses the font Arial and consistently uses a flush-left margin. We are an engineering company. My job consists in preparing documents and letters for customers. Everything I read in manuals and from technical writers directs me to use one space after periods. I find that it works very well, except occasionally, when an extra space helps readability. Knowledgeable engineers have embraced the one space use as being consistent with the font design and automation of reports. Others are unpleased with the one space, they think they have difficulty reading. (I, too, had an adjustment period which I forced myself to endure until it became automatic to read easily.) We are preparing technical information. What do you think? Thanks for your wonderful support and especially the quick answers. I greatly appreciate your service.

A. The view at CMOS 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.

So, in our efficient, modern world, I think there is no room for two spaces after a period. In the opinion of this particular copyeditor, this is a good thing.