From Proto-Finnic *-ic'eC, consisting of the suffix *-icci plus an additional consonant. The additional consonant is only found in Finnish; the other Finnic languages have forms deriving from the vowel-final form. Cognate with Estonian -itsi, Karelian -ičči, Veps -iči.
- by, through; the suffix of the prolative, determines the channel by which something takes place.
Some grammars treat the prolative not as a separate case because it seems not to occur in concorded form (an adjective in PROLATIVE + a noun in PROLATIVE); allegedly only with nouns in this form (e.g. postitse - by post; puhelimitse - by phone; etc.). However, there is one example which makes the prolative pass the concord test and therefore make the PROLATIVE a case:
Hän hoiti asian pitkitse kirjeitse. -- He took care of the matter with a long letter.