Top 50 Obscure Plural Forms in English

English is a fascinating language with a rich history, and one of its quirks is the variety of ways it handles pluralization. While we’re all familiar with regular plurals like cats and dogs, there are many obscure and unusual plural forms that often stump even native speakers. Below is a list of 50 obscure or irregular plural words in English, some of which are borrowed from other languages, and others that have simply evolved in unique ways over time.

1. Alumnus → Alumni

  • From Latin: Alumnus refers to a male graduate, and alumni refers to a group of male or mixed graduates.

2. Bacterium → Bacteria

  • Singular: Bacterium; Plural: Bacteria. A common scientific term where the singular is rarely used in casual conversation.

3. Crisis → Crises

  • Singular: Crisis; Plural: Crises. An irregular plural formed by changing “is” to “es.”

4. Datum → Data

  • Singular: Datum; Plural: Data. In common usage today, data is often treated as singular, but technically it’s plural.

5. Focus → Foci

  • Singular: Focus; Plural: Foci. Used in mathematics and science.

6. Cactus → Cacti

  • Singular: Cactus; Plural: Cacti. Latin-derived plural, though cactuses is also accepted.

7. Octopus → Octopodes

  • Singular: Octopus; the original plural from Greek is octopodes, but octopi (from Latin rules) and octopuses are also used.

8. Radius → Radii

  • Singular: Radius; Plural: Radii. Found in geometry and anatomy.

9. Nucleus → Nuclei

  • Singular: Nucleus; Plural: Nuclei. A common term in both science and mathematics.

10. Appendix → Appendices

  • Singular: Appendix; Plural: Appendices. Often seen in academic writing.

11. Thesis → Theses

  • Singular: Thesis; Plural: Theses. Common in academic contexts.

12. Phenomenon → Phenomena

  • Singular: Phenomenon; Plural: Phenomena. Used in science and philosophy.

13. Stimulus → Stimuli

  • Singular: Stimulus; Plural: Stimuli. Seen in psychology and biology.

14. Analysis → Analyses

  • Singular: Analysis; Plural: Analyses. Another example of the “is” to “es” rule.

15. Criterion → Criteria

  • Singular: Criterion; Plural: Criteria. Often misused as criterions.

16. Antithesis → Antitheses

  • Singular: Antithesis; Plural: Antitheses. Another “is” to “es” pluralization.

17. Corpus → Corpora

  • Singular: Corpus; Plural: Corpora. Used in linguistics and biology.

18. Index → Indices

  • Singular: Index; Plural: Indices. Common in mathematics and academia.

19. Matrix → Matrices

  • Singular: Matrix; Plural: Matrices. Frequently used in math and computer science.

20. Stratum → Strata

  • Singular: Stratum; Plural: Strata. Used in geology and sociology.

21. Vortex → Vortices

  • Singular: Vortex; Plural: Vortices. Found in physics.

22. Axis → Axes

  • Singular: Axis; Plural: Axes. Common in geometry and physics.

23. Louse → Lice

  • Singular: Louse; Plural: Lice. An example of a complete internal vowel change.

24. Goose → Geese

  • Singular: Goose; Plural: Geese. Another internal vowel change.

25. Man → Men

  • Singular: Man; Plural: Men. A common irregular plural.

26. Woman → Women

  • Singular: Woman; Plural: Women. Another irregular plural that changes vowels.

27. Mouse → Mice

  • Singular: Mouse; Plural: Mice. A complete internal vowel shift.

28. Foot → Feet

  • Singular: Foot; Plural: Feet. A common irregular plural.

29. Tooth → Teeth

  • Singular: Tooth; Plural: Teeth. Another example of vowel change.

30. Child → Children

  • Singular: Child; Plural: Children. An irregular plural with a suffix change.

31. Ox → Oxen

  • Singular: Ox; Plural: Oxen. An old English plural form.

32. Die → Dice

  • Singular: Die; Plural: Dice. In gambling and gaming, the singular and plural are often confused.

33. Brother → Brethren

  • Singular: Brother; Plural: Brethren. Used in religious or formal contexts.

34. Cow → Kine

  • Singular: Cow; Plural: Kine. An archaic plural, rarely used today.

35. Penny → Pence

  • Singular: Penny; Plural: Pence. Used in British currency.

36. Cherub → Cherubim

  • Singular: Cherub; Plural: Cherubim. Found in religious texts.

37. Seraph → Seraphim

  • Singular: Seraph; Plural: Seraphim. Also found in religious contexts.

38. Formula → Formulae

  • Singular: Formula; Plural: Formulae. Common in math and science.

39. Stadium → Stadia

  • Singular: Stadium; Plural: Stadia. Used in architectural and sports contexts.

40. Medium → Media

  • Singular: Medium; Plural: Media. Used in communication and art.

41. Syllabus → Syllabi

  • Singular: Syllabus; Plural: Syllabi. Common in education.

42. Memorandum → Memoranda

  • Singular: Memorandum; Plural: Memoranda. Used in business and legal writing.

43. Appendix → Appendices

  • Singular: Appendix; Plural: Appendices. Common in academic and medical contexts.

44. Automaton → Automata

  • Singular: Automaton; Plural: Automata. Used in robotics and mechanics.

45. Bacillus → Bacilli

  • Singular: Bacillus; Plural: Bacilli. Found in microbiology.

46. Genus → Genera

  • Singular: Genus; Plural: Genera. Used in taxonomy.

47. Curriculum → Curricula

  • Singular: Curriculum; Plural: Curricula. Common in education.

48. Oasis → Oases

  • Singular: Oasis; Plural: Oases. Found in geographical contexts.

49. Parenthesis → Parentheses

  • Singular: Parenthesis; Plural: Parentheses. Used in grammar and writing.

50. Quiz → Quizzes

  • Singular: Quiz; Plural: Quizzes. An example of doubling the consonant before adding “es.”

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