Q. I work in curriculum. I need to be able to spell out large numbers so as to model how to read numerals correctly. I can find rules for when to hyphenate whole numbers, but I can’t find any for hyphenating decimals. Specifically, I need to know when to hyphenate the words to the right of the decimal (tenths/hundredths, etc.). Please advise. Thanks so much!
A. That’s a challenging question! Let’s start with a few numbers and how we would suggest spelling them out—on both sides of the decimal point:
1,357,201.5: one million, three hundred fifty-seven thousand, two hundred one and five tenths
1,357,201.58: one million, three hundred fifty-seven thousand, two hundred one and fifty-eight hundredths
1,357,201.580: one million, three hundred fifty-seven thousand, two hundred one and five hundred eighty thousandths
1,357,201.5803: one million, three hundred fifty-seven thousand, two hundred one and five thousand eight hundred three ten-thousandths
Note that we’ve used commas between groups of numbers to the left of the decimal point but not to the right, which reflects how the numbers are grouped (and punctuated) as digits. But we’ve treated the numbers to the left of the decimal point as a single value rather than as a series (which might require a serial comma before and).
The rules for hyphenation are the same on both sides of the decimal point (see CMOS 7.96, section 1, “numbers, spelled out”). But note that Chicago’s preference for hyphenating simple fractions doesn’t apply to five tenths in the first example above, which simply names the number in the tenths place. It does, however, apply to an ordinal fraction like ten-thousandths:
five-tenths (a simple fraction)
five tenths (the number in the tenths place; see first example above)
ten-thousandths (the ten-thousandths place; see last example above)
See also CMOS 7.96, section 1, “fractions, simple.” Finally, note that some writers add and when spelling out certain numbers that include hundred (three hundred and fifty-seven thousand; two hundred and one); Chicago omits this and (see CMOS 9.5).