Proofreading Tips: Cacti or Cactuses? A Guide to Irregular Plurals

As a proofreader (or an aspiring one), you should know not all plurals are formed by adding -s to the end of a singular noun. You’ve probably also learned some irregular plurals by heart. But are there rules you should follow here? And how do they work if so?

In this post, we’ll look at some common types of irregular plurals and explain how they work.

Wolfs or Wolves? Words Ending with -f or -fe

Some words that end in -f have regular endings. These include:

And all words that end in a -ff take an extra -s in plural form:

However, some words that end in -f or -fe are pluralised by adding -ves instead. For instance:

This -ves ending is most common in words derived from Old English, although there are exceptions (such as roofs above). Some words can even take either ending, such as scarves/scarfs and wharfs/wharves (although the -ves endings tend to be more common).

Interestingly, the plural of dwarf was always dwarfs until JRR Tolkien decided to change it to dwarves in the Lord of the Rings books. Today, dwarfs is still the most common spelling in all contexts, but dwarves is an acceptable alternative when referring to the fantasy creatures.

However, if your client is referring to people with restricted growth, a condition sometimes known as dwarfism, the correct spelling is always the regular plural dwarfs.

Volcanos or Volcanoes? Words Ending with -o

Some words ending in -o can become regular plurals by simply adding an -s:

Others, though, can only take an –es in the plural form:

And at least a few can take either -s or -es in the plural form:

Some rules of thumb can help here. For instance, the following usually take a simple -s ending:

• Words ending in a vowel plus -o, including words ending in -oo (e.g., patios, ratios, zoos).
• Shortened forms of longer words (e.g., combos, intros, pantos).
• Proper nouns (e.g., The collection has three Picassos).
• Words borrowed from Spanish and Italian (e.g., tacos, concertos).

However, there are some exceptions, so it’s best to check a dictionary if you’re unsure.

Crisis or Crises? Words Ending with -is, -ex, or -ix

Words that end in -is often have ancient Greek roots. As such, they follow the convention of changing -is to -es in the plural, such as in the following terms:

Words ending in -ex or -ix also often come from Greek. You can pluralize these by changing the suffix to -ices. However, most have acceptable alternative spellings:

But, plenty of words ending in -ex take only regular endings in the plural:

Phenomenons or Phenomena? Words Ending with -on

There are two common English words that end in -on that always have irregular plural forms: phenomenon and criterion. Both take an -a ending as plurals:

Phenomenon → Phenomena

Criterion → Criteria

This can also apply to other words derived from Greek. But such words may also accept a standard English plural form. For instance, the plural of lexicon is usually lexicons, but it can sometimes be lexica. The plural of tetrahedron, on the other hand, is usually tetrahedra, but the English -s form of tetrahedrons is usually accepted as a variant spelling.

In addition, some Greek-derived words only take an -s ending in modern English, such as polygons and neurons. As such, it is important to be careful, and to double check the standard spelling if you’re ever unsure, when dealing with plurals of words that end -on.

Stadium or Stadia? Words Ending with -um

Similar to the Greek plural form discussed above, words derived from Latin that end in -um are sometimes spelled with an -a suffix in their plural forms. However, with a few exceptions (notably stratum, which is always spelled strata when pluralised), most of these words can also be spelled with an English -s ending. For instance:

In such cases, one spelling will usually be more common than the other. For instance, the most common plural of stadium is stadiums in modern English. But the standard plural form of millennium is still millennia, using the traditional Latin ending. There isn’t really a rule of thumb to follow here, so it can be helpful to check if you’re not sure which form is the standard plural spelling (typically, the standard plural form will be listed first in dictionaries).

To make matters worse, there are certain Latin-derived terms that follow their own rules!

Traditionally, for example, the plural form of the singular datum is data. And datums would always be wrong. But data is now often used as a singular term rather than a plural (e.g. The data is conclusive). As such, it is important to consider whether data is singular or plural when proofreading so you can ensure that the sentence it appears in is grammatical.

Similarly, the traditional plural of medium is media. And this is the correct plural in most cases. But mediums is used when referring to psychics. Again, then, this makes it important to consider the context and how a word is being used when making corrections.

Cacti or Cactuses? Words ending with -us

Words ending in -us also often come from Latin. Formal or technical words of this kind will usually take a Latin-style ending, changing -us to -i in the plural:

Other words, predictably, can be spelled either way:

And some words ending in -us always have regular plurals:

An interesting variation on this theme is octopus. It has so often been pluralized as octopi that this has become an acceptable spelling even though octopus doesn’t come to English from Latin. Rather, following the Greek roots of the word, the traditional plural would be octopodes. But the most common plural form in modern English is octopuses.

Irregular Nouns with Changed Vowel Sounds

For some words, the plural forms involve changing the vowel sound:

Occasionally, a word might change more substantially, such as with mouse and mice (although mouses is also a rare variant sometimes used for the computer equipment). Usually, these will be common words that should be easy for any native English speaker to spot. But if you’re unsure about a specific word, you can always check it in a dictionary.

Words That Stay the Same

Finally, a small number of words retain their spelling in the plural form. These include:

Even here, though, there are some tricky terms to look out for! Fish, for example, is almost always the correct plural form when referring to more than one fish. But some people will use fishes to indicate they’re referring to more than one type of fish. As such, as ever, you should consider the context in which a word is used when deciding whether it is correct.

How to Approach Irregular Plurals when Proofreading

As a proofreader, then, how should you handle irregular plurals? When a word has one correct spelling, all you need to do is correct it as you would with any other spelling error. And if you’re unsure whether an irregular plural is correct, you can look it up in a dictionary.

But what about words with more than one accepted spelling, such as octopi/octopodes/octopuses? In such cases, you have a few options:

• If your client is using a style guide, check for guidelines on irregular plurals.
• Where one spelling is more common than the other and there isn’t a reason for using the rare version, use the standard spelling for your client’s chosen dialect (you can use Google’s Ngrams tool to check how frequently different spellings come up in published writing).
• If there’s no reason to favour one spelling over another, simply check for consistency (and if your client uses both equally, leave a comment suggesting to pick one or the other).

The key, as always, is ensuring your client’s writing is clear throughout. And, ultimately, how a word is pluralised can sometimes be a matter of preference!

Becoming A Proofreader

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