First, to make things clear, I am not talking about reading of Kanji like 人(ひと, にん or じん) or 月(がつ, げつ or つき) All theese readings is in the list of On’yomi or Kun’yomi when you check the Kanji page of 人 and 月. There may be some rules or pattern behind, and I see there were already discussion on that, but they are specific for each of these Kanji.
What I would like to ask is I found sometimes the reading of the Kanji is twisted to add Dakuten. For example, pronounciation of 国 in 四国, 米国, 外国, 国王 or 全国 are all こく , but that in 中国 and 天国 is ごく. If you take a look at the Kanji page of 国, the reading of ごく is not in the list. So it seems it is some rule behind to twist the reading こく to ごく. I see that is also happening on 中 (from ちゅう to じゅう) and 大 (from たい to だい) as well. I keep missing the review on these vocabs and it would be great if there are some rules for these twists. So, are there any rules behind these twists? And if yes, is there a term for this twist (for further google search)?
