Is there any rule explaining why certain second-declension nouns and adjectives with a nominative -er ending drop the e when declined (e.g. ager, liber, pulcher), and why others keep it (e.g. puer, socer, līber)? (A fuller list of these nouns.)

That it has to do with a phonetic pattern seems to be contradicted by phonetically similar words falling in either category, e.g. adulter, adulterī vs. culter, cultrī.

  • 2
    I bet there was a historical development such that just as there are 2nd declension nouns that decline like 1st, there were also 2nd that declined like 3rd (so that originally it was ager, agris) but over time they shifted to 2nd declension endings. Mera est conjectura, sed non incredibilis videtur. Mar 2 '16 at 16:02

This is well-known and virtually all good grammars discuss this (as vowel syncope).

Genetivus singularis helps us reconstruct the original nom.sg. form (synchronically), that's why we learn nouns in Latin in two forms, nominativus singularis and genetivus singularis.

Type A. No change, the vowel was present in all forms.

Nom. sg. pueros > *puers > puer

Gen. sg. pueri

Type B. Originally, there was no e in Nom. sg.

Nom. sg. agros > *agrs > ager

Gen. sg. agri

Data from Old Latin and other Indo-European languages (especially Ancient Greek) confirms this.


  1. consonant + *-ros, e.g. SAKROS > sacer (cf. ferus);

  2. vowel + *-ros (in trisyllabic words, short penult), e.g. *vesperos > vesper (cf. sincerus)


For further details see Weiss 2009/2011, Chapter 23; Baldi 2002: 313; Meiser 1998, §55.1, and Sihler §74.4.


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