[Editor’s note: This post relies on the 16th edition of CMOS, which is no longer supported.]

When—and how often—to spell out abbreviations

Frequently, writers to the “Chicago Style Q&A” express the belief that when an abbreviation* is introduced in a document, it must be introduced once and once only (when the term first appears) and that thereafter the spelled-out term must never be used again.

The writers are usually seeking permission to spell out the term more than once, whether in a heading, a note, or in the text. They want to help the reader but are reluctant to break what they read as “the rule” at CMOS 10.3.

But there is no such rule! Any term may be spelled out or abbreviated as needed. Let’s take a closer look at section 10.3.

This guideline, like most CMOS guidelines, tries very hard not to sound like a rule. Phrases like “almost always,” “generally,” “roughly,” “usually,” and “may be” are meant to suggest that flexibility is key. CMOS 10.3 never says that it is mandatory to introduce an abbreviation once and only once. It does say that less familiar abbreviations must be spelled out on first occurrence—but that is just common sense.

The reason CMOS contains a guideline at all for introducing abbreviations is that the editors here at the University of Chicago Press have read a few too many manuscripts in which writers (1) never spell out an obscure abbreviation, (2) spell out a long, awkward term dozens of times, sometimes even more than once in a sentence, instead of introducing an abbreviation, (3) spell out a term and give the abbreviation every time it’s mentioned, or (4) spell out a term and introduce an abbreviation but then never use the abbreviation.

For this reason we recommend at CMOS 10.3 that

• unfamiliar abbreviations should be explained at first mention,
• very familiar abbreviations (UK, Dr., FBI, etc.) need not be spelled out at all,
• abbreviations in nonscience, nontech settings should be kept to a minimum,
• there’s little point in introducing an abbreviation and then never using it, and
• if there are a lot of abbreviations, a list of them is helpful to readers.

What’s ideal is to give the readers a well-written text, not annoy them with repetition, and remind them what a given abbreviation stands for whenever they might have forgotten it or missed when it was introduced.

Simply common sense.

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*In its general guidelines, CMOS uses the term abbreviation to cover abbreviations, initialisms, and acronyms.

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