Q. Dear style gurus, the rule is to always use the numeral with “percent,” as in “1 percent, 100 percent, etc.” Our question concerns “zero percent.” I say it should be spelled out, because your numeral rule applies to “numbers ONE through one hundred.” My co-worker says, nope, you’ve got to use 0. Who’s right? What’s the rule?

A. Our rule is that all percentages and decimal fractions are set in numerals, in humanistic as well as scientific copy. The only exception is for the beginning of a sentence:

Zero percent of American-born participants in a recent poll could remember where they were—not to mention what they were doing—when Theodore Roosevelt was charging up San Juan Hill.

The ideal income tax, some people say, would be 0 percent of net income.

The seventeenth edition of CMOS makes this clear: Chicago’s general rule is to spell out zero through one hundred. See paragraph 9.2.