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

§18. Latin Nouns of the Third Declension

By far the largest and most important category of Latin nouns is the 3rd declension, a group of words comprising all three genders and showing a great diversity of form. Your first reaction may be one of dismay, since this declension has no consistent word-ending, like the -a, -us, and -um of the 1st and 2nd, and appears to have little predictability of any kind. You’ll soon learn that the situation is really not that chaotic, since the 3rd declension does operate on regular and consistent principles.

One subtype of the 3rd declension, a group of nouns ending in -ex or -ix, has given rise to several English words that are pure Latin in form:


When we pluralize these rather technical words, we get English forms like vertices, appendices, matrices, and the like. These, too, are pure Latin, and illustrate the –es plural that is regular for all masculine and feminine nouns of the 3rd declension. (The hybrid plurals indexes and vortexes are correct English, but they differ in usage from indices and vortices.) Since Latin plural endings are always added to the BASE of a word, we can deduce that the base forms of index and vortex are indic– and vortic-. In dealing with the 3rd declension, we’ll find that the base can be quite different from the nominative (vocabulary) form of the word. This is a major contrast with the 1st and 2nd declension, where we merely lop off the final ending to get the base.

Before pursuing this line of enquiry, let us first examine another 3rd declension subtype that may occur unchanged in English—nouns ending in –or. Here is a sampling, presented as Latin words:

  1. arbor, clamor, clangor, color, favor, fervor, honor, labor, odor, rumor, savor, vapor, vigor
  2. error, horror, languor, liquor, pallor, squalor, stupor, terror, torpor, tremor
  3. actor, factor, doctor, creator, spectator, victor, pastor

The abstract nouns in group 1 are traditionally spelled –our in English, reflecting their French transmission; but ever since the reforms of the great lexicographer Noah Webster (1758-1843), they have been spelled –or in American usage.[1] For historical reasons, the parallel words in group 2 keep their original Latin form even in British spelling. Group 3, a list that could be greatly extended, consists of AGENT NOUNS—that is, they identify the person performing a verbal action. We’ll see more of groups 2 and 3 when we deal later with the Latin verb. As a general subtype, the Latin –or noun is a particularly easy form, since the NOMINATIVE AND THE WORD BASE ARE IDENTICAL.

There is another subtype of the third declension where the base of the word can be regularly inferred from the nominative form. For a noun like finis (“end”), we merely remove the -is ending to get the base fin-. Similar to finis are civis (“citizen”), hostis (“enemy”), testis (“witness”), vestis (“garment”), and navis (“ship”).

By and large, however, when we are learning a 3rd declension noun—even only for purposes of English word derivations—we must learn TWO FORMS of that noun. It is not enough to know that the Latin word for “king” is rex; we must know also that the base of this word is reg-, if we are to recognize regal as an adjective that means “kingly.” The base reg-, in fact, is considerably more important to us than the nominative form rex. In our tabular word lists, 3rd declension nouns will be presented in this fashion:

rex, regisking

The first of these forms, rex, is the NOMINATIVE or subject case, which is the standard vocabulary entry. The second form, regis, is the GENITIVE case (very much like the English possessive form, “king’s”). The reason we’ll be using the genitive is because it is the most dependable way of finding the BASE of every Latin noun or adjective. REMOVE THE is ENDING OF THE GENITIVE FORM, AND YOU WILL HAVE THE BASE OF ANY 3RD DECLENSION NOUN.

In the vocabulary list that follows, do not be surprised that there are many different nominative endings; that is the way the 3rd declension works. As you study the genitive forms, try always to associate the Latin base with a memorable English derivative. Occasionally, as with pars, partis (base = part-) or origo, originis (base = origin-), the base may even provide the obvious English derivative.

labor, laboris work pes, pedis foot
finis, finis end urbs, urbis city
rex, regis king vox, vocis voice
lex, legis law crux, crucis cross
ars, artis skill hospes, hospitis host, guest
pars, partis part miles, militis soldier
mors, mortis death origo, originis source, origin

As we saw above, it really isn’t necessary to list two forms for labor or finis, since these bases are predictable; but the second (genitive) form is needed for all the other words. Spelled like its original Latin source-word, English crux (plural cruces) means a “problem” or “critical point.” Most of the other nominative forms, however, are of little use to us. To repeat, it is the BASE forms like leg-, ped-, and hospit– that will play a key role in word derivation, both in Latin and in English. As we progress through the course, we’ll come to understand the links in form between Latin mort– and English mortify or immortality, between Latin urb– and English suburban or conurbation.

Once you feel familiar with Table 3.1, turn to the next group of 3rd declension nouns. Their gender, in every instance, is NEUTER, as opposed to the MASCULINE and FEMININE 3rd declension words that we’ve seen so far. There are two reasons for pointing out this fact. First, neuter nouns of the 3rd declension fall, for the most part, into easily recognized subtypes. Second, it may be useful to know that 3rd declension neuter nouns—like all Latin neuter words—have a plural ending in –a ; just think of our English phrase per capita, which means “by heads”).

caput, capitis head corpus, corporis body
cor, cordis heart tempus, temporis time
lumen, luminis light genus, generis race, kind, sort
omen, ominis omen opus, operis work, task
nomen, nominis name onus, oneris load, burden

The shock here, no doubt, will be the discovery that there is more than one class of Latin nouns ending in –us. This 3rd declension subtype has nothing to do with words like animus or campus, and mustn’t be confused with that 2nd declension group.

The secret of learning these noun bases and remembering their spelling is to think of their adjective derivatives in English: capital, cordial, luminous, ominous, nominal, corporal, temporal, general, and onerous. That trick doesn’t work for opus, but there we can think of the English verb operate. The nominative form opus is an English word, of course, used mainly to identify a work of music; in Latin, it tends to suggest the tangible product of work, rather than the process (labor). Corpus, omen, and genus are other 3rd declension neuter nouns that have entered English without change; of these, only genus regularly keeps its original Latin plural—genera.


  1. The Canadian spelling of these words, like many aspects of Canadian life, is a little indecisive. Canadian newspapers have been using the -or forms since the nineteenth century, but Canadian schools—at least, those schools that still teach spelling—cling to the British -our preference. One system is no more “correct” than the other. There are some -our/-or words, like harbour and neighbour, that are actually of Germanic origin. Others, like endeavour, are derived from Latin, but not from Latin -or nouns.


Icon for the Creative Commons Attribution 4.0 International License

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.

Share This Book