Devi and Vrkis feminines
In Vedic Sanskrit, the devī and vṛkīs inflections are two types of inflection of feminine ī-stems exhibiting distinct apophony patterns.
Vṛkīs
The distinguishing feature of the vṛkīs inflection is that the ī always has the Vedic accent except in the vocative case, and the nominative singular has the desinence -s like non-feminine words. Indeed, while vṛkīs-words are overwhelmingly of the feminine gender, there are a few members of the class that belong to the masculine gender or are gender indeterminate: rathī- "wain-driver, charioteer" (often applied to Agni, who trafficks sacrificial offerings and divine boons between mortals and immortals).
The inflectional type is usually accepted to reach back into Proto-Indo-European times, with an exact correspondence of Sanskrit vá¹›kÄ«s and Old Norse ylgr, both meaning "she-wolf", first described by Karl Verner in 1877 (see Verner's law). The distinction between devÄ« and vá¹›kÄ«s dies out in during the Vedic period and PÄṇini is unaware of it, classifying Ä«-stems by accentuation (vá¹›kÄ«s-words are a subset of NÄ«S).
One formation that has been diachronically connected with the vá¹›kÄ«s inflection is Cvi, which in PÄṇini's grammar of Classical Sanskrit refers to a formation where an Ä« is added to a nominal stem and compounded with a verbal root ká¹› "to make", as "to be" or bhÅ« "to become", resulting in a factitive verb where the Ä«-stem is indeclinable and used like a preverb. For example, grÄmÄ«bhÅ« "to get possession of a village", from grÄma "village".
Devī
The devÄ« inflection exhibits an ablaut pattern different from the vá¹›kÄ«s inflection. PÄṇini does not make the distinction, classifying the Ä«-stems by their accentuation (devÄ« words may be NiiN, NiiP or NiiS).