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Q. In CMOS 9.48, on numbered places of worship, shouldn’t the example “Twenty-First Church of Christ” read as “Twenty-first Church of Christ”? Why is the second number uppercase?
A. The word “first” in “twenty-first” is an adjective, so it gets a capital F as the second part of a hyphenated compound in the name of an organization or the title of a work—or in a street name or whatever else would normally be capitalized. See CMOS 8.161, which also says specifically to capitalize the second element in a hyphenated spelled-out number. This advice was new to the 16th ed. (2010); until then, CMOS would have advised “Twenty-first,” but our editors agreed that the convention for lowercase wasn’t strong enough to continue to allow for an exception to the rule about adjectives. “Twenty-first” isn’t wrong, then, but it’s no longer Chicago style.