The standard seems to imply that there is no restriction on the number of definitions of a variable if it is not odr-used (§3.2/3): Every program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program; no diagnostic required. It does say that any variable can't be defined multiple times within a translation unit (§3.2/1): No translation unit shall contain more than one definition of any variable, function, class type, enumeration type, or template. But I can't find a restriction for non-odr-used variables across the entire program. So why can't I compile something like the following: // other.cpp int x; // main.cpp int x; int main() {} Compiling and linking these files with g++ 4.6.3, I get a linker error for multiple definition of 'x'. To be honest, I expect this, but since x is not odr-used anywhere (as far as I can tell), I can't see how the standard restricts this. Or is it undefined behaviour?