Corresponding to the Etruscan ablative I (Rix 1985: 127 f., Wallace 2008: 47). Like the pertinentive, the ablative is built with genitive forms / adjectives of possession as base (Rix 1985: 127). According to Rix, the pre-historical form is *-si-s, the genitive's i-element, protected from apocope, causing umlaut of the stem vowel before being syncopated. (Alternatively, elision of the genitive's s and subsequent vowel merger.) Hence, -a > -ais > -es, -e > -eis, -us > -uis, -is > -is; -es and -is are formally identical with genitives in -s to stems in -e and -i. Pronouns behave differently due to the different accent situation (see θeis). It may be argued that the ablative is historically a genitive to the genitive (genitiuus genitiui in Eichner's terminology); differently Steinbauer 1999: 71, who analyses -s as a postposition (without a possessive derivation as base).
The ablative I itself is attested in only one inscription, inflecting the enclitic pronoun (θeis) and possibly the ambiguous form θaukis. Its function in the text is not quite clear. Neither the ablative II -las (based on the genitive II -l(a)) nor the apparently archaic ablative III (with null suffix) are attested in Raetic.