日本語 Japanese Ask Questions Thread 8

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英語でも日本語でもOK
日本の方からのご協力も大歓迎です

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Like >>137, a question also partially inspired by KLK, but it's one that I've always wondered myself.

A question about the Romanization of 四天王. I've always wondered why it's Romanized as "Shitennou" or "Shiten'nou", rather than "Shitenou". Why does that second "n" pop in?

Also, why do some Romanizations have apostrophes? Like "Shiten'nou" or "Shin'ichiro"? Is there some rule with -n that I'm not seeing? It doesn't seem to be a standard to add an apostrophe either.