They are essentially based on their Proto-Indo-European ancestors, and the Latin cardinal numbers are largely sustained in the Romance languages.
which also carried over into the anno Domini system and Christian dating, e.g. annō post Chrīstum nātum centēsimō for AD 100.
A special series of numeral adjectives was used for counting these, namely ūnī, bīnī, trīnī, quadrīnī, quīnī, sēnī, and so on.
The sēnārius was a kind of metre consisting of six iambic feet commonly used in spoken dialogue in Roman comedy.
The silver dēnārius is often mentioned in the New Testament, and was stated to be the day's pay in the parable of the Labourers in the Vineyard.
[8] The numeral ūnus < Old Latin oinos ‘one’, with its cognates Old Irish óen ‘one’, Gothic ains ‘one’, Ancient Greek οἴνη oínē ‘ace on dice’, and the first part of Old Church Slavonic inorogŭ ‘Unicorn’, harks back to Proto-Indo-European *Hoi̯-no-s.
[17] The masculine nominative/accusative forms dŭŏ < Old Latin dŭō ‘two’ is a cognate to Old Welsh dou ‘two’,[16] Greek δύω dýō ‘two’, Sanskrit दुवा duvā ‘two’, Old Church Slavonic dŭva ‘two’, that imply Proto-Indo-European *duu̯o-h1, a Lindeman variant of monosyllabic *du̯o-h1, living on in Sanskrit द्वा dvā ‘two’, and slightly altered in Gothic twai ‘two’, German zwei ‘two’ etc.
The accusative forms dŭōs m., dŭās f., the genitive dŭom, classical dŭōrum m./n., dŭārum f., and the dative/ablative dŭōbus m./n., dŭābus f., are original Latin formations replicating nominal declension patterns; at times, duo stands in for other case forms, especially when combined with invariant numerals, e. g. duo et vīgintī ‘twenty-two’, duodētrīgintā ‘twenty-eight’.
[15][18] Most Romance languages sustain an invariant form developed from the masculine accusative duōs > Spanish, Catalan, Occitan dos, French deux, Romansh duos, dus; Italian due seems to preserve the feminine nominative duae (or may have evolved from the feminine accusative duas).
The masculine and feminine nominative form trēs ‘three’ and its cognates Gothic þreis ‘three’, Greek τρεῖς treîs ‘three’, Sanskrit त्रयः trayaḥ ‘three’ are based on Proto-Indo-European *trei̯-es; the original accusative form trīs, matching Umbrian trif, Gothic þrins, Old Irish trí,[16] Greek τρίνς tríns < Proto-Indo-European *tri-ns, was being superseded from preclassical Latin onward.
[17] The invariant numeral quattuor ‘four’ does not fully correspond to any of its cognates in other languages, as Oscan petora ‘four’, Greek τέσσαρες téssares ‘four’, Old Irish cethair ‘four’, Gothic fidwôr ‘four’, Lithuanian keturì ‘four’, Old Church Slavonic četyre ‘four’ point to a Proto-Indo-European base *kʷetu̯or-, that should appear as *quetuor in Latin; the actual -a- has been explained as epenthetic vowel emerging from a zero-grade *kʷtu̯or-.
The geminate -tt- might have been established to compensate the fluctuating quality of succeeding -u- between non-syllabic glide and full vowel apparent since Old Latin; in the postclassical form quattor this sound is dropped altogether, and in most Romance languages the second syllable is subject to syncope, which then is compensated by an additional vowel at the very end of the word, as in Spanish cuatro, Portuguese quatro, Italian quattro, French, Occitan, Catalan quatre, Romanian patru.
[15][18][17] The cardinal number quīnque ‘five’, with its cognates Old Irish coíc ‘five’, Greek πέντε pénte ‘five’, Sanskrit पञ्च pañca ‘five’, leads back to Proto-Indo-European pénkʷe; the long -ī-, confirmed by preserved -i- in most Romance descendants, must have been transferred from the ordinal quīntus ‘fifth’, where the original short vowel had been regularly lengthened preceding a cluster with a vanishing fricative: quīntus < *quiŋxtos < *kʷuiŋkʷtos < *kʷeŋkʷ-to-s.