Irregular plurals

Writing Tips >> Irregular plurals

Irregular plurals are often seen in scientific writing. When a noun is made plural by changing or keeping its spelling rather than attaching an inflectional suffix (i.e. -s or -es), the plural noun is considered irregular. Irregular plurals usually have Greek or Latin origins.

Some nouns change the spelling when becoming plural. The following table shows examples of irregular plurals which change in different ways morphologically.

 Change Singular Plural Change Singular Plural
-ex -ices index indices -on -a criterion criteria
  vertex vertices   phenomenon phenomena
-is -es analysis analyses -um -a medium media
  axis axes   quantum quanta
  basis bases   spectrum spectra
  diagnosis diagnoses   stratum strata
  synthesis syntheses -us -i locus loci
  thesis theses   nucleus nuclei
-ix -ices appendix appendices   radius radii
  matrix matrices   stimulus stimuli


Some nouns do not change the form when becoming plural. Two examples are shown below. 

Singular Plural
series series
species species


In addition to changing or keeping the spelling, a noun may have two acceptable plural forms – an irregular plural and a regular plural. Below are some examples.

Singular Irregular Plural Regular Plural
apparatus apparatus apparatuses
focus foci focuses
formula formulae formulas
schema schemata schemas


To ascertain the correct forms of irregular plurals, dictionaries should be consulted.

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