Functions as a phonologically conditioned alternative form (allomorph) of the suffix -culum, from Proto-Indo-European *-tlom. It possibly developed from a phonological rule that deleted the initial stop of the suffix when it came after a stop, or from simplification of geminates (e.g. *spek-tlo- > *spek-klo-). However, Sen 2015 argues that instrumental ending -ulum is derived from the neuter of the etymologically distinct suffix -ulus (found in some agent nouns, such as figulus (“potter”)) from Proto-Italic *-elos.^[1] Compare the feminine variant -ula.

• (Classical Latin) IPA^(key): [ʊ.ɫũː]
• (modern Italianate Ecclesiastical) IPA^(key): [u.lum] (stressed on the antepenult)

-ulum n (genitive -ulī); second declension

1. Used to form a noun of instrument or means from a verb.

   ‎amicio + ‎-ulum → ‎amiculum

   ‎specio + ‎-ulum → ‎speculum

   ‎cingo + ‎-ulum → ‎cingulum

Second-declension noun (neuter).

See the etymology of the corresponding lemma form.

-ulum

1. inflection of -ulus:
   1. nominative/accusative/vocative neuter singular
   2. accusative masculine singular

• Clackson, James, Indo-European Word Formation: Proceedings from the International Conference, 2002.