-lus

Etymology
From syncope of (source of Classical Latin ), from, thematized from.

Usage notes
The suffix -lus is added to a noun to form a diminutive of that noun. As with other Latin diminutive suffixes, the gender of the diminutive regularly matches the gender of the base noun.

This form of the diminutive suffix is attached mainly to first- or second-declension nouns or adjectives whose stem-final vowel (-a- or -o-) is preceded by /r/, /l/ or /n/. Depending on declension and gender, this corresponds to words that end in the nominative singular with -ra, -er/-rus, -rum; -la, -lus, -lum; or -na, -nus, -num. As the result of syncope, the /r/, /n/, or /l/ of the stem is brought into contact with the /l/ of the diminutive suffix and is assimilated to it, forming geminate /ll/.

When the /r/, /n/, or /l/ in the base word is preceded by a short vowel or consonant, the diminutive usually ends in -ellus/-a/-um or -illus/-a/-um. (More rarely, some end in -ullus/-a/-um.) The vowel before /ll/ in the diminutive may differ from the vowel used in the base word. In the oldest formations, these vowels likely developed from regular sound changes. However, it is likely that many diminutives ending in -ellus and -illus were formed more recently than the stage when these sound changes occurred and took their vowels based on analogy with existing pairs of words. Thus, the endings and  had already started to be extended as suffixes in their own right in pre-literary times.

However, if for the sake of morphological analysis we treat diminutives formed from bases in -ra-/-ro-, -na-/-no-, or -la-/-lo- as ending in the suffix, the following generalizations can be made about the use of /ell/, /ill/, or other vowels + /ll/:

Bases with stems that end in -ra-/-ro- preceded by either the short vowel /e/ or by another consonant typically form diminutives in /ell/ (per Strodach, the only exception attested in Republican Latin is from ; a second exception attested in Imperial Latin is / from ):

Bases with stems that end in -na-/-no- preceded by a long vowel retain the quality, and probably also the quantity, of the long vowel before the geminate /ll/:

Bases with stems that end in -na-/-no- preceded by the short vowel /i/ typically form diminutives in /ell/ (with the exception of from  ). This /i/-/e/ alternation likely developed initially from the sound change of vowel reduction, by which original short vowels /e/ or /a/ regularly turned into /i/ in word-medial open syllables but into /e/ in word-medial closed syllables:

Bases with stems that end in -na-/-no- preceded by another consonant most often form diminutives in /ill/, sometimes in /ell/:

Bases with stems that end in -la-/-lo- preceded by the short vowel /u/ sometimes form diminutives in /ell/ and sometimes in /ill/:

For some words, the base but not the diminutive shows the effect of sound changes on consonant clusters:
 * (base originally had -bn-, which turned into -mn- by regressive assimilation to nasality)
 * (base originally had -tn-, which turned into -nn- by regressive assimilation to nasality)
 * (base originally had -ksl-, which turned into -◌̄l- by lenition of s-final clusters before voiced consonants)

A few third-declension nouns with stems ending in -r- or -n- may form diminutives with -lus:
 * Examples:

More often, third-declension nouns form diminutives with, such as and , also from  and.