Hyphens, En Dashes, Em Dashes

Q. In your hyphenation table (CMOS 7.89), why is “mid-twentieth” in “mid-twentieth century” hyphenated? Shouldn’t it be closed up as “midtwentieth century” or have an en dash instead of a hyphen?

A. That’s a good question! The fourteenth edition (1993) did advise an en dash: “mid–twentieth century.” But we decided as of the fifteenth (2003) that this was just too fussy. The hyphenated form lends itself better to compound modifiers, as in “mid-twentieth-century furniture.” So when the less common noun phrase is used, we prefer to retain the hyphen: “mid-twentieth century.” A similar logic has discouraged us from advising “midtwentieth century”—though we do recommend “midcentury.”