> **Possible Duplicate:**  
> [When did it become correct to add an ‘s’ to a singular possessive already ending in ‘s’?](https://english.stackexchange.com/questions/2964/when-did-it-become-correct-to-add-an-s-to-a-singular-possessive-already-ending)  
> [Which singular names ending in “s” form possessives with only a bare apostrophe?](https://english.stackexchange.com/questions/79081/which-singular-names-ending-in-s-form-possessives-with-only-a-bare-apostrophe)  

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Which one is correct?  
I thought the latter would be correct but apparently the former is always used; why?

###Edit:

Another (confusing) example: [Charles'(s?) law](http://en.wikipedia.org/wiki/Charles%27s_law)