UTC In Hepburn romaji or all romaji Consider the following It is stated that any kanji beginning with d in romaji can be doubled So zi in Nihon shiki and Kunrei and di in Nihon shiki and di or zi in Kunrei can be doubled Except that and are both ji in Hepburn and therefore can not be doubled You need to use the kana not the romaji spelling of the kana to explain which kana can be used with the sokuon because not all romaji systems transliterate all the kana the same Of course you may know that the sokuon is never used with or but someone just learning Japanese like myself won t 07 56 15 February 2015