Numbers in Indexes

The most memorable characteristic of an index is the arrangement of the entries in alphabetical order*. It makes it easy to find what you’re looking for, and even the picky differences between word-by-word and letter-by-letter arrangement aren’t noticeable to a casual user.

But what about numbers? Since they’re not letters and therefore not obviously subject to alphabetization, many beginning indexers are unsure what to do with them.

If there aren’t many numbers, the common solution endorsed by the Chicago Manual of Style, Nancy Mulvaney’s Indexing Books, and the American Society for Indexing’s Best Practices, is to sort the numbers as if they’re spelled out. So the 3M company would go in the Ts, where “three M” would fit. 1984 would go under the Ns. This goes for numbers in the middle of a heading, too, like “September 11, 2001” or “War of 1812.”

This mostly works. How many numbers are considered “not many” is subject to indexer interpretation. Three? Five? (Sort in the Ts and Fs, respectively.) Whatever you choose, if there are more than “not many,” you should create a separate section for numerals (or numerals and symbols), and chuck ’em in there. Sorted in ascending arithmetic order, of course. My general rule is if there few enough entries that the separate section would look silly in comparison to the rest of the index, I sort them as if spelled out. Most dedicated indexing software, as well as Microsoft Word’s indexing function, will allow indexers to insert hidden sort codes to force the numbers to sort.

But what if, as my kids are fond of asking? What if there are two entries that only differ by number, such as “Constitution of 1791” and “Constitution of 1795”? Pure spelled-out sort would put the later date first, as “five” comes before “one”. And do you spell it as “seventeen hundred and ninety-one,” “seventeen ninety-one,” or (shudder) “one thousand, seven hundred, ninety-one?” In cases like this, it makes more sense to sort the numbers arithmetically (chronologically). Anything else could look like a mistake.

Numbers (and symbols) are of special interest in scientific and computer books, and the rules differ slightly. For chemical compounds, the number is ignored. If there are multiple compounds that differ only by the initial number, they’re sorted arithmetically within the letter grouping for the alphabetically part of the compound name. So “2-fluoroaniline” goes before “3-fluoroaniline,” in the Fs (even though “three” goes before “two” in the alphabet).

Numbers can also be filed in ascending size order, as in a list of computer chips, or in chronological order, as in the French Constitution examples above. Legal cases also are sometimes differentiated only by dates.

Indexing is a process that depends on both knowing the rules and knowing when to break them. The indexing maxim, “It depends,” is the answer to so many questions for a reason. Awareness of the audience, the subject matter,  and plain common sense are all important when writing an index.

*Most of the time. Chronological sort or ascending locator order can occur in subheadings, especially in the case of biographies, where the problem of “marriage” sorting after “death” in subheadings is a common example.