I’ve written on this topic many times before (e.g. here), but too many out there either haven’t gotten the message or willfully resist it. So here we go again ...
If you are writing in a proportionately spaced font (i.e. almost anything besides Courier*), you need only one space—not two—at the end of each sentence. The latest and (in my mind) most authoritative voice saying so is Matthew Butterick. Besides being a lawyer, Matthew has credentials most lawyers lack in this area. He has a visual-arts degree from Harvard, where he learned traditional letterpress printing and digital font design. He worked as a font designer for several years before entering law school and becoming a practicing litigator. He explains on his web site and in his new book why, when writing in a proportionately spaced font, one space is enough and two is too many. So if you’re still {sigh} putting two spaces after every sentence, please read what Matthew has to say about it.
Matthew is not alone in his view. Every authority I’ve consulted on this question agrees with him, including the following:
- The Chicago Manual of Style §§ 2.12 and 6.11 (15th ed. 2003).
- The U.S. Seventh Circuit’s Requirements and Suggestions for Typography in Briefs and Other Papers, page 5.
- Ruth Anne Robbins, Painting With Print: Incorporating Concepts of Typographic and Layout Design into the Text of Legal Writing Documents, 2 J. Assn. Legal Writing Directors 108, 129 (Fall 2004) (recommended by the U.S. Seventh Circuit).
- Kenneth A. Adams, A Manual of Style for Contract Drafting, §§15.50–15.55 (2d ed. ABA 2008).
If you still insist on putting two spaces after every sentence despite using a proportionately spaced font, be my guest. But I’d like to know if anyone out there can cite any authority to support such a practice.
__________
* If, on the other hand, you’re using your computer to produce text in Courier or another monospaced font, put two spaces after every sentence, as you learned in typing class. But please consider re-examining your choice of fonts.
p.s. (31 Jan. 2011): For some high-minded debate on this topic, check out First Draft.

You can add to your list of authority, Bryan Garner, editor of Black's Law Dictionary. I can't cite anywhere specific for the rule, but I asked him at one of his seminars and he agreed with the one space rule.
Posted by: Matt T. | 26 January 2011 at 12:06 PM
Thanks, Matt. I looked for something about that by him, but didn’t find anything. Bryan did write the foreword for Matthew’s book, however, and declared Matthew almost infallible on the subject of typography.
Posted by: Ray | 26 January 2011 at 01:05 PM
Texas Law Review Manual on Usage and Style (used by many attorney in Texas) says one or two is fine. (Garner "help[ed]" them according to the intro.) They note that the Review itseld uses two spaces.
I don't understand why the rule isn't the same regardless of the font used. I can read monospaced fonts with only one space in between sentence, just as easily as I can read proportional fonts. And the rivers of white space created by two spaces applies regardless of font type. (I guess I'm asking why there was ever a two space rule to begin with.)
Personally, I like the two spaces and how the sentences are set off a little more. But I'm switching to one spaces since the professionals say that is an easier read.
Posted by: James L. | 26 January 2011 at 01:29 PM
I will always use 2 spaces. 1 space has always looked too crowded to my eyes, and it always will. In this particular (as well as on "sexism"), I must part ways with Bryan.
Posted by: Aaron | 26 January 2011 at 02:31 PM
For those think that two spaces "just looks better," consider this: Every professionally typset book, newspaper, and magazine uses one space. For websites, even if an author uses two spaces in the HTML (like Aaron's comment, above), Web browsers automatically convert those two spaces to one. Every published work you have ever read has one space. Yet no one has said "I just can't read this novel; its single spaces annoy me." Why should legal writing be different?
Where every leading authority -- Chicago Manual of Style, Seventh Circuit, Butterick, Robbins, and Bringhurst's "Elements of Typographic Style" -- recommends one space, try to find an authority that champions two spaces. Most people only have one: their high-school typing teacher. For lawyers who rely on authority, that particular authority is not very persuasive.
Posted by: Damien | 28 January 2011 at 04:35 PM
The Second Edition of The Redbook: A Manual on Style by Bryan Garner recomends one space after periods. Look to Rule 4.12, located on page 83.
Posted by: Ryan | 04 February 2011 at 11:50 AM
Ryan: Thanks for spotting that. I thought I remembered Bryan saying something about that somewhere, but I couldn’t find it in his usage dictionaries or the indices to his other books.
Posted by: Ray | 04 February 2011 at 06:07 PM
I know it's silly but I like 2 spaces cuz then I can figure out where one sentence ends and another begins in cases like the following -
It was after 6:00 p.m. Mary needed to go to the store.
Also, in some legal writing where a party is referred to by initials only, it becomes confusing not to have the extra space where a sentence ends:
The evidence related to the charges against Stephen K. Rodriguez testified against the defendant.
Posted by: JDMom | 08 February 2011 at 11:34 AM
For me, this problem is not about the intellectual understanding of the concept of proportional fonts and the unnecessary second space; it is about my stupid left thumb that has been behaviorally modified to always tap twice after a period. I don't realize I am double spacing until I am halfway through a paragraph, so it works best for me to just write an entire draft and then use "Find and Replace" (in Word) to find all double spaces and reduce them by half.
Posted by: Kasey Libby | 25 February 2011 at 10:59 AM
I just worked on a pleading with co-counsel who used the 1-space rule. As a fan of Garner and trusting the others who advocate for the 1-space rule, I saw it as my chance to start the new wave here.
Then I started reading the document and adding in the spaces. It ended up looking way too crowded for my tastes. Instead of being a first attempt at change here, it just convinced me to always use 2 spaces.
Posted by: Cory | 19 August 2011 at 04:54 PM
Cory: Thanks for your comment. My only suggestion would be to explore the reasons behind the rules. Good typography is not just a matter of aesthetics—what some people somewhere think looks better. It’s a matter of legibility—ease of reading—which can be scientifically measured. Take a look at Ruth Anne Robbins’s article, Painting With Print (linked to in the above post). It changed the way I think about typography.
Posted by: Ray | 19 August 2011 at 05:38 PM