How to Form Nouns of Latin's Third Declension

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Latin is an inflected language, meaning the endings of Latin words change to indicate their function in a sentence (e.g. subject, direct object, indirect object). Latin nouns are declined in one of five declensions, the third of which is the most widely used in the language. Luckily, by the time beginning students encounter the third declension, they have experience with the quirks and irregularities of the first two.

Unlike the previous two declensions, third declension nouns may be any of the three genders, masculine, feminine or neuter. Every third declension noun is characterized by the genitive ending –is as in regis (masculine), virtutis (feminine), and corporis (neuter). However, unlike the characteristic nominative singular ending of the first (–a) and second (–us, –um) declensions, the third declension has no characteristic word ending. Thus, it is vital to memorize the gender of all nouns as well as the genitive form of the word so it is obvious to which declension it belongs and how it is properly declined.

Third Declension Forms for Masculine and Feminine Nouns

Third declension nouns are formed just as in the previous two declensions; the endings are added to the base of the word. However, one complexity of third declension nouns is that the base sometimes changes from the nominative to the other cases. This is why learning the nominative form of a noun is not enough. Always memorize the genitive of every noun so their forms are immediately familiar when encountered in text. Luckily, masculine and feminine nouns are declined exactly the same so no new forms must be memorized. Unfortunately, similar forms mean no clue is given as to a noun’s gender. Again, memorization is the key. A masculine noun is declined as follows:


nom. rex

gen. regis

dat. regi

acc. regem

abl. rege


nom. reges

gen. regum

dat. reginbus

acc. reges

abl. regibus

Notice that the base for all cases is different from the nominative singular. It may help to know that rex was originally regs in earlier Latin. However, the –gs sound is so similar to –x that it changed by the time Classical Latin was spoken. Remember that a language is spoken first and written later. This is especially true of ancient languages at a time when the majority of the population was illiterate.

The singular forms of third declension masculine and feminine nouns all have different endings, so there is no confusion for beginning students other than memorizing the new endings. Notice that in the genitive plural, the familiar –um is present, similar to the –orum of second declension nouns and –arum or the first declension. The nominative and accusative plural has the same form, like second declension neuter nouns. The plural dative and ablative forms are similar, as they are in the previous two declensions, although the endings are different here in the third declension.

Third Declension Forms for Neuter Nouns

Third declension neuter nouns prove a bit more difficult for beginning students because although they often change form from the nominative to the genitive, they also have the same form for nominative and accusative in the singular. Pay close attention to the gender of a third declension noun to avoid common mistakes. For example:


nom. corpus

gen. corporis

dat. corpori

acc. corpus

abl. corpore


nom. corpora

gen. corporum

dat. corporibus

acc. corpora

abl. corporibus

Notice that the forms are similar to the masculine and feminine forms with two exceptions. The nominative and accusative singular have the same form. The accusative form reverts back to the nominative form. This often gives students trouble because they become used to the shift in spelling for the other cases and forget that the accusative must look exactly like the nominative. Also, notice the characteristic –a ending for the nominative and accusative plural, a form familiar from neuter nouns of the second declension. The word corpus often gives students trouble in the genitive singular because the –orum looks like the ending for a second declension noun. The stem of the word is corpor– and the ending is –um. It is only coincidental that the form looks like the –orum ending from the second declension.


The third declension is the most often used declension in Latin. Consequently, it contains many exceptions to the rule. Think of this like a hammer. The more a hammer is used, the more dings and dents it has from use. A rarely-used hammer remains more perfect in form with far fewer dents. With this in mind, it is essential to memorize the gender and genitive forms of all nouns so that they may be applied without hesitation and without having to leaf through notes or text to find the correct forms.

This post is part of the series: Latin Declensions

Since Latin is an inflected language, the endings of its words change to indicate their function in a sentence. Latin nouns and adjectives are declined into five basic cases. These nouns and adjectives belong to one of five declensions, each with its own set of rules and forms.

  1. How to Decline First Declension Latin Nouns
  2. A Primer on Latin’s Second Declension
  3. Learn How to Form Third Declension Latin Nouns
  4. Latin Declensions: Fourth Declension
  5. How to Decline Latin Words of the Fifth Declension