Why do I need this? Not every noun takes the usual -s or -es ending in their plural form. There are a handful of nouns with alternative plural forms, including very common words like ‘children’, ‘data’, ‘feet’, ‘series’, and ‘fish’.
Click on the ‘What’s the Difference?’ column to group the nouns according to their type of morphological change (which should help if you’re trying to memorize these plural forms) instead of viewing the words alphabetically.
| Single Form | Plural Form | What's the Difference? | Domain | |
|---|---|---|---|---|
| * | addendum | addenda | ending changes to 'a' from 'um' | documentation |
| aircraft | aircraft | single and plural forms are identical | engineering | |
| alumna | alumnae | ending changes to 'ae' from 'a' | people | |
| alumnus | alumni | ending changes to 'i' from 'us' | people | |
| analysis | analyses | ending changes to 'ses' from 'sis' | math & physics | |
| * | antenna | antennae | ending changes to 'ae' from 'a' | engineering |
| * | apex | apices | ending changes to 'ices' from 'ex'/'ix' | |
| appendix | appendices | ending changes to 'ices' from 'ex'/'ix' | documentation | |
| axis | axes | ending changes to 'ses' from 'sis' (but with 'x' instead) | math & physics | |
| ◊ | bacterium | bacteria | ending changes to 'a' from 'um' | plants & animals |
| basis | bases | ending changes to 'ses' from 'sis' | ||
| bison | bison | single and plural forms are identical | plants & animals | |
| buffalo | buffalo | single and plural forms are identical | plants & animals | |
| * | cactus | cacti | ending changes to 'i' from 'us' | plants & animals |
| child | children | plural form takes an 'en' ending | people | |
| codex | codices | ending changes to 'ices' from 'ex'/'ix' | documentation | |
| * | concerto | concerti | ending changes to 'i' from 'o' | |
| crisis | crises | ending changes to 'ses' from 'sis' | ||
| ◊ | criterion | criteria | ending changes to 'a' from 'on' | |
| curriculum | curricula | ending changes to 'a' from 'um' | documentation | |
| ◊ | datum | data | ending changes to 'a' from 'um' | engineering |
| * | deer | deer | single and plural forms are identical | plants & animals |
| diagnosis | diagnoses | ending changes to 'ses' from 'sis' | medicine & psychology | |
| ◊ | die | dice | plural form takes an 'ice' ending | |
| ellipsis | ellipses | ending changes to 'ses' from 'sis' | documentation | |
| erratum | errata | ending changes to 'a' from 'um' | documentation | |
| *† | fish | fish | single and plural forms are identical | plants & animals |
| * | focus | foci | ending changes to 'i' from 'us' | math & physics |
| foot | feet | vowel-shift from 'oo' to 'ee' | ||
| * | formula | formulae | ending changes to 'ae' from 'a' | math & physics |
| * | fungus | fungi | ending changes to 'i' from 'us' | plants & animals |
| goose | geese | vowel-shift from 'oo' to 'ee' | plants & animals | |
| * | hippopotamus | hippopotami | ending changes to 'i' from 'us' | plants & animals |
| hovercraft | hovercraft | single and plural forms are identical | engineering | |
| * | index | indices | ending changes to 'ices' from 'ex'/'ix' | documentation |
| * | larva | larvae | ending changes to 'ae' from 'a' | plants & animals |
| * | locus | loci | ending changes to 'i' from 'us' | |
| ◊ | louse | lice | plural form takes an 'ice' ending | plants & animals |
| * | mage/magus | magi | ending changes to 'i' from 'us' | |
| man | men | plural form takes an 'en' ending | people | |
| ◊ | medium | media | ending changes to 'a' from 'um' | documentation |
| * | memorandum | memoranda | ending changes to 'a' from 'um' | documentation |
| moose | moose | single and plural forms are identical | plants & animals | |
| mouse | mice | plural form takes an 'ice' ending | plants & animals | |
| * | nebula | nebulae | ending changes to 'ae' from 'a' | math & physics |
| * | nucleus | nuclei | ending changes to 'i' from 'us' | math & physics |
| oasis | oases | ending changes to 'ses' from 'sis' | ||
| *§ | octopus | octopi | ending changes to 'i' from 'us' | plants & animals |
| ox | oxen | plural form takes an 'en' ending | plants & animals | |
| ◊ | panino | panini | ending changes to 'i' from 'o' | |
| ◊ | paparazzo | paparazzi | ending changes to 'i' from 'o' | people |
| ◊ | parenthesis | parentheses | ending changes to 'ses' from 'sis' | documentation |
| person | people | [other] | people | |
| * | phenomenon | phenomena | ending changes to 'a' from 'on' | |
| phylum | phyla | ending changes to 'a' from 'um' | plants & animals | |
| prognosis | prognoses | ending changes to 'ses' from 'sis' | medicine & psychology | |
| * | radius | radii | ending changes to 'i' from 'us' | math & physics |
| series | series | single and plural forms are identical | ||
| sheep | sheep | single and plural forms are identical | plants & animals | |
| * | shrimp | shrimp | single and plural forms are identical | plants & animals |
| spacecraft | spacecraft | single and plural forms are identical | engineering | |
| species | species | single and plural forms are identical | plants & animals | |
| stimulus | stimuli | ending changes to 'i' from 'us' | medicine & psychology | |
| swine | swine | single and plural forms are identical | plants & animals | |
| * | syllabus | syllabi | ending changes to 'i' from 'us' | documentation |
| synopsis | synopses | ending changes to 'ses' from 'sis' | documentation | |
| thesis | theses | ending changes to 'ses' from 'sis' | documentation | |
| tooth | teeth | vowel-shift from 'oo' to 'ee' | ||
| * | vortex | vortices | ending changes to 'ices' from 'ex'/'ix' | math & physics |
| watercraft | watercraft | single and plural forms are identical | engineering | |
| woman | women | plural form takes an 'en' ending | people |
Footnotes:
The morphological changes to the plural forms typically carry through when these words are a part of compound words, are hyphenated, adjoin with prefixes, etc. For example, the plural of ‘reindeer’ is ‘reindeer’, and the plural form of ‘fireman’ is ‘firemen’.
The ‘domain’ simply means the context in which one is likely to use or encounter these words. We believe putting these nouns into groups may help some learners to tackle the words in portions that make sense to them. The categories we’ve placed in this column are somewhat subjective and arbitrary, so teachers and learners are welcome to develop their own categories here.
Take advantage of every method, idea, tip, and trick we have to offer teachers. We’ll keep you informed of every new resource we release.