Words ending with a suffix "an" such as American, European, and Asian are a good example of referring people living in or born in a certain place. Likewise, the suffix "er" is also very common as in New Yorker and Hong Konger.
My question is if there is any rule in English regarding which suffix to add when we would like to describe residents in a country, state, city, etc. For example, how would you call people in Denver, Colorado? Is it Denverer, Denverian, or what?
Interestingly enough, a resident of Sydney seems to be called "Sydneysider." After all, I feel there is no fixed one rule. Is there any?