Q. When did CMOS first recommend one space (instead of two) after periods and colons? I found the one-space rule in the 15th edition, but I remember hearing somewhere that it goes back to the 13th edition. I’m trying to win arguments with people who claim it’s a “new” rule.
A. The sample typewritten manuscript page in CMOS 13 (1982) shows two spaces after a period; in CMOS 14 (1993), it shows one (fig. 2.1 in both editions). But CMOS 15 (2003) was the first edition to make an explicit recommendation for one space after a period (or a colon) in typed manuscripts. It’s a little more complicated for published documents. The 1st edition (1906) described a system of variable spacing that was the norm at the time. (For example, an “em quad” was recommended between sentences—three times the amount of space required between words.) By 1949, when the 11th edition was published, equal spacing was the rule: “The standard for composition [typesetting] such as that in the text of this book would be a 3-to-em space [a third of an em] . . . between words, after colons, after exclamation and interrogation points, and after periods ending sentences” (11th ed., p. 8). So “one space” is a relatively new convention for manuscripts but less new for published documents.