When I studied classic latin at school, I always wondered why there were only very few neuter nouns belonging to the fourth declension. Is there a historical reason for that? Could it be that many migrated to the second declension neuter or to the fourth itself as masculine/feminine?

My textbooks reported that only three 4th decl. neuter names are in common use, but there are few more I could find in the dictionary or on the internet, listed below.

  1. cornu, -us Horn
  2. genu, -us Knee
  3. veru, -us Spit
  4. gelu, -us Frost (also gelum, -i)
  5. pecu, -us Cattle (also pecus,-oris; pecus, -udis)
  6. testu, -us Earthenware lid (used as indeclinable testu; also testum, -i)
  7. tonitru, -us Thunder (also, tonitrus, -us; tonitruum, -i; tonitruus, -i)
  8. specus, -us Cave (generally masculine or feminine; neuter in poetry)

Are there more known?

  • 1
    I'm not sure if this is the actual reason, but fourth-declension neuter nouns are very close to being indeclinable, which means the case system isn't very useful for them. I'd expect the less common ones to either become indeclinable, or get reanalyzed as fourth masculine or second neuter.
    – Draconis
    Jan 2 at 16:45
  • There are lists in Wiktionary . They reject tonitru but add seru (which is "special" since genitive might be seru). Note that there are also not many pure f. (they say 40 but many are names, Greeks or "specials" in one respect or another. Also note they have 500+ m. but again, many are derivation from ppp or suffix -atus. Not sure how many "native"/"pure"
    – d_e
    Jan 2 at 19:20
  • @Draconis thank you, I thought about that and I always thought that in a way fourth and fifth declension were in someway "unnecessary" (my current understanding is that they are latin inventions, while the others were born from the PIE. Probably their origin is a topic for anohter question).
    – Davide
    Jan 3 at 10:50
  • @d_e Thanks, I was not aware of this resource in the Wiktionary. Seru has also a second declension counterpart serum, -i. Regarding Tonitru I attache some reference where I found it mentioned youtube.com/watch?v=FQCKBfZsRa0 (at 3:47), classicalliberalarts.com/classical-trivium/latin-grammar/… and augustinus.it/italiano/regole/regole.htm (searching "tonitru" in the webpage).
    – Davide
    Jan 3 at 10:55

1 Answer 1


I don't have a complete explanation, but it seems most likely to me that there weren't that many to begin with and unlike second or third declension neuters, Latin had hardly any productive processes that created new ones.

If you enumerate neuter nouns in the second declension, you will see that many of them are (or were) formed with productive derivational suffixes such as -ium or -ētum or -mentum or -culum. Likewise, in neuter nouns of the third declension, we repeatedly see the endings -men and -al/-ar/-āle/-āre and -us which all had at least some productivity as derivational suffixes at some point.

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