Notice that for certain morphemes that begin with the morae "chi" () and "tsu" (), their rendaku forms begin with the morae "ji" and "zu", spelled "precisely" in hiragana as and , which explains the use of these kana in contrast to the identically pronounced and (see "yotsugana"). This isn't a hard and fast rule, however, because it's relaxed in certain older compounds or names, especially those that are so consolidated that they could hardly be recognized as compounds anymore, but rather, as single words themselves.