A Proofreader’s Guide to Latin Plural Word Endings

From abacus to virus, lots of English nouns have origins in Latin. These words can pose a challenge for proofreaders, though, as many have two plural forms. Here, we’ll look at:

• Some common Latin plural word endings.
• The difference between traditional and English Latin plurals.
• The exceptions to the rules.

Common Latin Plural Word Endings

Common Latin word endings in modern English include:

Singular Ending	Plural Ending	Example
-a	-ae	Formula → Formulae
-ex or -ix	-ices	Appendix → Appendices
-on or -um	-a	Stadium → Stadia
-us	-i	Cactus → Cacti

These word endings follow the original Latin spelling conventions. But English usually forms plurals by adding “-s” or “-es” to a noun. As a result, many words with roots in Latin now have more than one plural form: we’ll call these the “traditional” and “English” plurals.

Traditional vs. English Latin Plurals

“Cactus” comes to English via Latin. As such, the traditional plural is “cacti”:

I have one cactus, but you have many cacti.

But “cactus” has been part of English for hundreds of years now. And in this time, many people – knowingly or otherwise – have favored a traditional English plural form:

I have one cactus, but you have many cactuses.

Other words with traditional and English plural forms include:

Singular Word	Traditional Plural	English Plural
Formula	Formulae	Formulas
Appendix	Appendices	Appendixes
Stadium	Stadia	Stadiums
Vertebra	Vertebrae	Vertebras
Radius	Radii	Radiuses
Aquarium	Aquaria	Aquariums

In most cases, both forms are acceptable. As a proofreader, then, you would not need to correct either “cacti” or “cactuses” unless there was an issue with consistency to address.

However, sometimes the “best” form to use may depend on the context:

• Traditional plural forms are most common in formal academic and technical writing. For example, you would expect to see “formulae” as the plural of “formula” in a research paper. “Formulas,” by comparison, may sound too informal for academic work.
• English plural forms are usually clearer in everyday writing and speech. For instance, most people would recognize “stadiums” as a plural of “stadium.” By comparison, “stadia” is less familiar so may confuse some readers (and it risks sounding elitist or pretentious).

When proofreading, then, think about which word ending would fit best for the document.

Exceptions to the Rules

While the guidelines above are useful, some Latin-derived terms only have one accepted plural spelling in English. Some are always pluralized with the English form (e.g., bonuses, viruses, campuses). Others always use the traditional Latin word ending (e.g., alga → algae).

In other cases, people mistakenly apply a Latin word ending. The most famous example is “octopus.” Derived from Ancient Greek, the traditional plural of this word in English would be “octopodes,” while the standard plural is “octopuses.” Nevertheless, because “octopus” ends in “-us,” many people mistakenly treat it as a Latin word and use the plural form “octopi.” And, ultimately, all three spellings are acceptable insofar as their meaning is clear.

If you’re unsure about the correct suffix for an English word with Latin (or Greek) origins, check a dictionary. You can also use Google’s Ngrams tool to check how often variant spellings are used. And don’t forget to check your client’s style guide for advice if they have one.

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