some nouns (such as borrowings from other languages, abbreviations, etc.) are not modified when they change number and case. this appears mostly when their gender appears to have no ending in any declension which suits the final part of the word: these are masculine names on vowels different from , female names on hard consonants (names like "trish" won't take the soft sign to go into third declension like native "mouse"). most borrowed words ending in russian in and stressed "а" are not declined: , (), etc. most abbreviations are undeclined (one exception is ). many people also think that georgian surnames on like () shouldn't be declined since they are originally something like russian possessive genitives.