日語 Japanese Ask Questions Thread2 質問

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Japanese - Ask questions thread
http://dis.4chan.org/read/lang/1174719097/l50

Is there a rule when two on-yomi kanji combine and they merge readings, like 作曲 is sakkyoku not sakukyoku. I kind of realized I can usually guess them too even if I've never seen or heard them read so there's got to be a rule.

Now that I think about it, is it when the last syllable's first letter matches up with the upcoming kanji's first letter?

Are there any exceptions or anything missed?