In quantum mechanics, spin multiplicity indicates the number of possible quantum states of a system with given principal spin quantum number S. The different states are distinguished by the spin projection quantum number Sz, which can take the values -S, -S+1, ..., S-1, S. Therefore, the multiplicity is 2S+1, where S is the number of singly occupied electrons multiplied by the Electron Spin Quantum Number, ms. A system with S=0 has exactly one possible state; it is therefore in a singlet state. A system with S=1/2 is a doublet; S=1 is a triplet, and so on.