In quantum mechanics each measurable physical quantity of a quantum system is called an observable which, for example, could be the position r and the momentum p but also energy E , z components of spin ( s z ), and so on. The observable acts as a linear function on the states of the system; its eigenvectors correspond to the quantum state (i.e. eigenstate) and the eigenvalues to the possible values of the observable. The collection of eigenstates/eigenvalue pairs represent all possible values of the observable. Writing ϕ ϕ i for an eigenstate and c i for the corresponding observed value, any arbitrary state of the quantum system can be expressed as a vector using bra–ket notation : | ψ ψ ⟩ ⟩ = ∑ ∑ i c i | ϕ ϕ i ⟩ ⟩ . The kets { | ϕ ϕ i ⟩ ⟩ } specify the different available quantum "alternatives", i.e., particular quantum states.