[Liumingke]

I've been trying to understand something. When is the kanji used. I've read for Noun for sure. Adjectives and verbs use what is called "okurigana" were the root word is kanji and the extend is in kana. So why is it that the word for dictionary じびき (kana) 字引 (kanji) in the book that I'm studying use the kana and not the kanji? Isn't dictionary a noun? Can someone clarify this for me? Is there no set rule? Thanks.