Q. Dear CMOS: In your style guide you write that there is no space between a number and the % symbol. But in math it’s normal to put a space between a number and a symbol. It’s also part of ISO and NIST and other standards. Why don’t you follow these standards?

A. The advice in CMOS to write, for example, “10%” rather than “10 %” (see paragraph 9.18) reflects how people generally use the percent symbol in English-language documents across a wide variety of genres, from fiction to technical reports. (But note that Chicago style for nontechnical settings is to spell it out: “10 percent.”) You’ll find this preference in the AP Stylebook (journalism), the AMA Manual of Style (medicine), and Scientific Style and Format (general sciences), among other guides.

Meanwhile, we are aware that a space is used between a numeral and the percent symbol in the International System of Units (SI), which is official throughout much of the world (see SI Brochure, 9th ed., 5.4.7, where this rule is stated). See CMOS 10.58, where we acknowledge this usage.

The discrepancy between SI, on the one hand, and Chicago and other styles, on the other, may simply be a matter of common usage versus usage in the sciences. It’s worth noting, however, that the SI Brochure, published by the International Bureau of Weights and Measures (Bureau International des Poids et Mesures, or BIPM), is primarily a French-language document; though it’s published simultaneously in English, the French text remains official (see SI Brochure, p. 124).

In French, spaces are used before certain marks of punctuation, including colons and semicolons, and between quotation marks (or, in French usage, guillemets) and the text they enclose. And though these conventions aren’t followed in the English version, perhaps they have something to do with the persistence of certain other spaces in the brochure—for example, the one in an expression like “5 °C,” which is commonly rendered in nontechnical contexts without a space.

Editors following Chicago style would support a strict interpretation of SI style in appropriate contexts—or query its use before making any changes. But we don’t all have to write like international scientists (as admirable as such a goal might be), nor does the space carry any special meaning in the sciences or elsewhere. As conventions go, this one is 100% (or 100 %) arbitrary, provided you’ve been consistent.

[Editor’s note: As a reader has kindly pointed out, a space between a number and a percent symbol or other unit represents multiplication. Our original point was simply that (for example) 100% and 100 % are generally intended to mean the same thing.]