If you are using libraries such as Wire in your own .c files or library then you need to #include <LibraryName.h> both in your library (where you're using the things it defines) and in your main sketch (which tells the IDE to include its folder in the search path for includes during compilation and to link in its .c / .cpp files). Otherwise you get errors like "'LibraryName was not declared in this scope"

 

 

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