In the mathematically rigorous formulation of quantum mechanics, the state of a quantum mechanical system is a vector ψ ψ belonging to a (separable) complex Hilbert space H . This vector is postulated to be normalized under the Hilbert space inner product, that is, it obeys ⟨ ⟨ ψ ψ , ψ ψ ⟩ ⟩ = 1 , and it is well-defined up to a complex number of modulus 1 (the global phase), that is, ψ ψ and e i α α ψ ψ represent the same physical system. In other words, the possible states are points in the projective space of a Hilbert space, usually called the complex projective space. The exact nature of this Hilbert space is dependent on the system – for example, for describing position and momentum the Hilbert space is the space of complex square-integrable functions L 2 ( C ) , while the Hilbert space for the spin of a single proton is simply the space of two-dimensional complex vectors C 2 with the usual inner product.