-si-

Etymology
Originally from the plural stem of.

Usage notes
is always placed before the case ending, but it is basically arbitrary whether it attaches to the singular or plural stem of any given word. It appears that it is always attached to the strong stem for nouns that have gradation. Even further, in some cases it is preceded by an unetymological, probably extracted by analogy from actually regular plural stems. The nominal form obtained as a result of attaching the locative case ending is considered to be in plural.

Only cases where the corresponding derivative does not exist as an independent noun can be considered to use, which has developed from the inflected forms of those derivatives by analogy, specifically.

Affix

 * 1)  reflexive affix which joins the prefix of a verb to the root