An empirical rule in the theory of atomic structure stating that for a principal quantum number n the number of electronic quantum states that can have the orbital quantum number l is 2(2l + 1). This rule describes the subshells of atoms. It was put forward on the basis of chemical evidence and spectroscopic evidence by John David Main Smith and independently on the basis of magnetic and spectroscopic evidence by Edmund Stoner in 1924. The Main Smith–Stoner rule is a consequence of the Pauli exclusion principle. The rule was one of the key developments that led to the enunciation of the Pauli exclusion principle in 1925.