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 by J. D. 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.