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To my understanding, Latin doesn't have a dual number at all. The adjectives duo "two" and ambō "both" have some special forms derived from the PIE dual (-ō, -ābus, -ōbus), but are considered irregular rather than part of any real pattern.

However, from an answer to an unrelated question:

By the way, another heterogeneous noun with a similar story behind it is frēnum "rein", pl. frēnī. In this case the Latin masculine plural ending continues what was in PIE terms not a plural at all, but a neuter dual.

This points to another trace of the dual number which I hadn't heard about before.

Are there any other irregularities or fossilized forms in Latin which derive from the dual number in PIE or Proto-Italic?

Joonas Ilmavirta
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Draconis
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3 Answers3

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The -ī of vīgintī "20" is originally a dual ending, the same one as in frēnī (PIE *-ih₁). This is why the ending of vīgintī is different from that of the other tens (trīgintā etc.)

TKR
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According to W.M. Lindsay The Latin Language: An Historical Account of Latin Sounds, Stems, and Flexions (p. 253), octo could also be a dual in form, with the sense of 'two sets of four'. He sees a comparison with ambo and duo because of the ending ō-sound.

I don't get why it would mean 'two sets of four' though.

piscator
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    If the ō-sound is enough, then Cicero must mean "two chickpeas". I hope the sound and eight being an even number were not his only reasons to propose this. – Joonas Ilmavirta Dec 06 '16 at 21:57
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    @JoonasIlmavirta. IE *oḱtō is the dual of a noun the singular of which survives as Avestan ašti- “span, four-fingers length”, as was pointed out by W.B. Henning, Transactions of the Philological Society 1948, p. 69. (Yes, this brilliant article is exactly one page long). – fdb Dec 06 '16 at 23:16
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It looks like a couple of neuter nouns of the fourth declension might have nominative singular forms derived from PIE duals. (Note that the nominative neuter ending -u might have been pronounced either as -ŭ or -ū; it seems that we don't have any clear evidence either way for the quality of the vowel in this context.) A form -ū with a long vowel could be explained as deriving from PIE dual *-uh₁; the dual form is mentioned as a possible source of a Latin form in De Vaan's entries for genu "knee" ("PIE *ǵen-u(-h₁ dual), *ǵen-u- (obl.)") and cornu "horn" ("If ū is original, cornu may reflect the nom.acc.dual ending *-uh₁; especially for cornū *'pair of horns' > 'horn'").

Asteroides
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  • This is interesting. I never realised how two of the most common -u nouns normally grow in pairs! And I've always found the -u ending odd (although I might have thought of a dropped -d à la illud, istud). – Cerberus May 05 '19 at 22:17
  • Cf. dative cornū (I think this exists in addition to cornui?). – Cerberus Jun 04 '21 at 18:24
  • @Cerberus: I'm not really sure about the etymology or frequency of dative -ū, but it apparently shows up on all kinds of fourth-declension nouns, not just neuters: How did the fourth declension neuter dative singular become different from the non-neuter ending? – Asteroides Jun 06 '21 at 03:33
  • Oh, had forgotten all about that question! It seems I had already voted for your answer there as well. – Cerberus Jun 06 '21 at 22:07
  • Weiss convincingly has the neuter athematic dual as **-ih₁* instead, but suggests the -ū in the fourth declension neuter nominative is due to the neuter plural ending **-h₂* and terminal laryngeal loss in pausa later creating long/short variation there. This created ambiguity that was resolved by the singular always being unetymologically long and the plural getting a new ending -ua analogous with the other declensions. – Cairnarvon Jun 10 '23 at 13:51