Rendaku in 一本気 but not in 本気 As I'm encountering more words with rendaku, I thought I started to understand it, but I came across an example that does not make sense to me. Why is voiced in and , but is voiced in ? In other words, why is there no rendaku in ? From other questions, I see that there is some unpredictability about rendaku… but those words are really close to one another.

Rendaku occurs when two independent words come together to form a compound. I think is a compound, while and are not. (Just because they are analyzable as + and + doesn't mean they are compounds. and are not independent words, unless you mean "a book" by .) Also, Sino-Japanese words don't normally participate in rendaku. In that sense, is exceptional.