All of the WikiMedia projects are closely related, and support each others' operations in various ways. If you stay involved with Wikipedia, the chances are good that sooner or later you will find yourself participating to some degree in one or more of Wikipedia's sister projects. If and when this happens, you will naturally want the same username there that you have on Wikipedia. Many Wikipedians maintain a single user identity throughout all the WikiMedia wikis. Even if you don't ever go to the sister projects, you don't want somebody else using the same username as you, potentially causing identity confusion. Therefore, create an account identical to your Wikipedia account on each of the Wikimedia projects before someone else chooses your name as their own. Even if you never need those accounts, they'll be ready and waiting in case you ever do. See m:Help:Unified login for information on how to do this quickly and easily.
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Here they are: Wikipedia's sister projects