The complementary idea is that of an intensive variable.
Suppose there is one piece of substance whose quantity is n and another piece whose quantity is m. Let V be an extensive variable. Let the first piece have a value V(n) of the extensive variable. Let the second piece have a value V(m) of the extensive variable. Then, if the two pieces are put together to form a piece with mass n + m, then their corresponding variable is
From equation (1) it can be deduced that