All quantum-mechanical particles possess an intrinsic spin s (though this value may be equal to zero). The projection of the spin s on any axis is quantized in units of the reduced Planck constant, such that the state function of the particle is, say, not ψ ψ = ψ ψ ( r ) , but ψ ψ = ψ ψ ( r , s z ) , where s z can take only the values of the following discrete set: s z ∈ ∈ { − − s ℏ ℏ , − − ( s − − 1 ) ℏ ℏ , … … , + ( s − − 1 ) ℏ ℏ , + s ℏ ℏ } .