Hyphens, En Dashes, Em Dashes

Q. In “number + noun” of the CMOS hyphenation table, you say “Hyphenated before a noun, otherwise open.” You include the following examples: “a one-and-a-half-inch hem” and  “one and a half inches.” As “inch” is a noun and “one and a half” is a number/quantity, why not “one-and-a-half inches”?

A. That part of the table is explaining what to do when a “number + noun” modifies another noun. In “one and a half inches,” inches is not modified by a “number + noun” phrase; it is merely modified by a number: one and a half. Therefore no hyphens. After all, we don’t hyphenate phrases like “a hem of two inches.” In phrases like “a one-and-a-half-inch hem,” the noun hem is modified by a “number + noun” phrase: “one-and-a-half-inch,” which is therefore hyphenated, as we would hyphenate “two-inch hem.”