-culum

• (Classical Latin) IPA^(key): /ku.lum/,
• (modern Italianate Ecclesiastical) IPA^(key): /ku.lum/, (stressed on the antepenult)

From -culus (diminutive suffix).

-culum

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

From (with anaptyxis) Proto-Italic *-klom, from Proto-Indo-European *-tlom, from *-trom. Compare stabulum, which comes from a similar suffix *-dʰlom. Despite the resemblance, ōsculum (which besides is never found in the form **ōsclum) and other diminutive nouns do not contain this suffix.

• -clum, -crum, -ulum

-culum n (genitive -culī); second declension

2. suffix used to form some nouns derived from verbs, particularly nouns representing tools and instruments

The main form of this suffix is -culum, but various alternative forms exist. Most tend to be found in particular phonological contexts:

• -crum can be found only if /l/ is present somewhere earlier in the word, as in lavācrum. It developed from Proto-Italic *-klom by long distance dissimilation (compare -ālis and its allomorph -āris). The non-dissimilated form -culum can also be found after /l/ in some words, such as liāculum.
• -trum is found whenever the suffix occurs immediately after /s/, as in haustrum. In addition, it is found in some words that contain a liquid /r/ or /l/ somewhere earlier, such as arātrum, tālitrum; this type of formation seems to have been old and unproductive in Classical Latin. It was inherited from the Proto-Indo-European variant form *-trom. Apart from the limited occurrence of the ending in inherited Latin vocabulary, it has been used to coin neologisms, at first with influence from the cognate Greek instrument noun suffix -τρον (-tron), later (in Neo-Latin) with influence from modern English inanimate agent nouns ending in -er or -or (such as computer) and similar formations in other modern European languages.
• -ulum is found whenever the suffix occurs immediately after a velar plosive (spelled c or g), as in cingulum. It originates either from simplification of *-tlom after these consonants, or from the neuter version of the originally distinct suffix -ulus (found in some agent nouns, such as figulus (“potter”)) from Proto-Italic *-elos.^[1]
• -rum is found after two stems that contain /l/ and end in a plosive: scalprum and fulcrum. It seems to originate from either *-tlom (with liquid dissimilation as in -crum) or *-trom, with simplification after a labial or velar plosive (as possibly in -ulum).

The suffix -bulum (dissimilated form -brum) is etymologically related and semantically similar; there are no obvious conditions for when it is used instead.

Most of these suffixes have variant first-declension feminine forms, such as -cula, -tra, -ula, -bula, -bra (as in pavīcula, mulctra, dēcipula, sūbula, dolābra). Variant second-declension masculine versions of these endings are much less common, but are attested in a few nouns, such as culter, scalper (a variant of scalprum), arāter (a rare variant of arātrum), and the plural rastrī.

Second-declension noun (neuter).

From -āculum (by rebracketing of the first-conjugation thematic vowel -ā-):

• Catalan: -all
• French: -ail, -aille
• Italian: -acchio
• Portuguese: -alho
• Spanish: -ajo

From -ī̆culum:

• Spanish: -ijo

• The Proto-Indo-European Instrument Noun Suffix *-tlom and its Variants, Birgit Anette Olsen, 1988.

• Clackson, James, Indo-European Word Formation: Proceedings from the International Conference, 2002
• Philip Baldi, The Foundations of Latin, 2002, pp. 304-305