The four quantum numbers describe the quantum mechanical properties of atomic orbitals. The principal quantum number (n) describes the main energy level or shell. The secondary quantum number (l) gives the sublevel or subshell and describes the shape of the orbital. The magnetic quantum number (ml) describes the orientation of an orbital in space. The spin quantum number (ms) describes the direction an electron is spinning and can have values of +1/2 or -1/2. Together, the four quantum numbers uniquely define the atomic orbital of an electron.