Chapter 3: The Latin Noun (Declensions 3, 4, 5)

§20. Latin Nouns of the Fourth Declension

The 4th declension consists of a relatively small but fairly important group of nouns, many of which were derived originally from Latin verbs. You may be perplexed to learn that they all have the ending -us; superficially, therefore, they can be confused with 2nd declension nouns like focus and animus or with 3rd declension neuter nouns like corpus and onus. However, there is one very useful rule-of-thumb for the 4th declension that will seldom let you down:

IF ANY LATIN -us NOUN HAS AN ENGLISH DERIVATIVE WITH A -u- BEFORE THE FINAL SYLLABLE, IT IS ALMOST SURE TO BE A 4TH DECLENSION LATIN WORD. To illustrate, if you are confronted with an unfamiliar –us noun—say, manus (“hand”)—and you can think of an English word in –ual like “manual,” assume that the word belongs to the 4th declension.

This rule works because the –u– vowel clings to the Latin base in forming Latin derivatives; and the Latin spelling is regularly maintained in English. For this reason, we shall say that the BASE of manus is manu-. (Contrast 2nd declension foc-us, E foc-al; 3rd declension corpus, corpor-is, E. corpor-al. )

A few 4th declension nouns appear unchanged in English: status, sinus, census, consensus, hiatus, apparatus. If you should want to pluralize any of these words in English, and you mean to follow Latin practice, you will not change the word in spelling—the Latin plural of census is census. It would be correct to say, in English, “one apparatus, two apparatus;” but it would also be acceptable to anglicize and say, “two apparatuses.” Whatever you do, don’t say “two apparati,” since that is neither Latin nor English.[1]

Latin manus has no simple noun derivative in English, though it is, of course, the source of French la main. As the French gender reveals, the Latin word was feminine, though most 4th declension nouns were MASCULINE. The following table presents a small sampling of these nouns; you will meet quite a few more when we come to examine the Latin verb, since most 4th declension nouns are derived from verbs.

gradus step, grade sinus curve, fold
manus (F.) hand situs position, site
ritus ceremony, rite spiritus breath, spirit

Our us rule-of-thumb works for all the words here. On the analogy of manual, just think of English gradual, ritual, sinuous, situate, and spiritual. Therefore, consider these Latin noun bases to be gradu-, ritu-, sinu-, situ-, and spiritu-.

Though Latin sinus could mean any curve or fold, English has limited the meaning of the word to a fold in the facial bone structure. That is an excellent example of the semantic change known as SPECIALIZATION (§15). The sine in trigonometry is a derivative of the same word. Latin situs is the source of English site; and if an artifact is still to be found in its original position, it is said to be in situ (a pure Latin phrase).


  1. Exactly this mistake was made by CBC commentator Brian Williams, who referred to “the four apparati of rhythmic gymnastics,” in an Olympic telecast on August 9, 1984. O tempora, o mores!


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Greek and Latin Roots: Part I - Latin by Peter Smith (Estate) is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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