In quantum mechanics a state space is a separable complex Hilbert space. The dimension of this Hilbert space depends on the system we choose to describe. The different states that could come out of any particular measurement form an orthonormal basis, so any state vector in the state space can be written as a linear combination of these basis vectors. Having an nonzero component along multiple dimensions is called a superposition. In the formalism of quantum mechanics these state vectors are often written using Dirac's compact bra–ket notation.