Q. In fiction, when a character reads off a hotel room number, would it be in numbers or spelled out? “Room 305, down the hall.” Or “Room three oh five, down the hall.”

Q. I’m currently editing a novel and having difficulty discerning whether Chicago would spell out temperatures or use numerals. CMOS 9.13 offers this example of the general rule for physical quantities: “Within fifteen minutes the temperature dropped twenty degrees.” But elsewhere in the Manual you use numerals: “the phrase freezing point denotes 32 degrees Fahrenheit or 0 degrees Celsius” (CMOS 5.250, under “connote; denote”) and “consisting of two geometric angles that, added together, take up 90 degrees” (CMOS 5.250, under “compliment; complement”). Could you please offer clear simple guidance as to how temperatures should appear in fiction? Thanks!

A. There are at least two principles at work in these two questions.

First, though numbers are often spelled out in dialogue (to help readers understand how they would be spoken), that doesn’t mean numerals are never used. Fictional room 305 would almost never be encountered in the real world as “room three oh five” or “room three hundred five” (or “three hundred and five”), least of all on an actual hotel room door. So “room 305” is the best option, even in dialogue; it’s how room numbers are known.

The second principle is precision. Passing mentions of temperatures, whether in dialogue or narration, would be spelled out in Chicago style: “Brrr, it must be ten degrees below zero out here!” (or “Brrr, it must be ten below out here!”). But freezing points and geometric angles represent exact measurements, and numerals are often the best way of communicating these in ordinary prose.

In dialogue, however, spelling out exact quantities suggests a different kind of precision—another meaning of spell out is to make something clear—so words would work at least as well as numerals:

“Freezing point is zero degrees Celsius,” he announced.

“I can draw a perfect forty-five-degree angle,” she bragged.

You might make an exception, however, if the character is referring to an actual numeral somewhere:

“The thermometer says 32 degrees,” I said, squinting at the display.

That “32” might help the reader imagine the scene, making the dialogue seem more realistic (much as writing “305” in the previous example would).

So numerals can work in dialogue for expressions that would always be written with numerals or when a character is referring to actual numerals; otherwise, it’s usually best to spell them out. In narrative, Chicago’s general rules for numbers apply—subject to editorial discretion. For more on this topic, see “Numbers in Creative Writing” at CMOS Shop Talk.