In calculations, one often encounters expression like ⟨ ⟨ 0 | T { ϕ ϕ ( x ) ϕ ϕ ( y ) } | 0 ⟩ ⟩ or ⟨ ⟨ Ω Ω | T { ϕ ϕ ( x ) ϕ ϕ ( y ) } | Ω Ω ⟩ ⟩ in the free or interacting theory, respectively. Here, x and y are position four-vectors, T is the time ordering operator that shuffles its operands so the time-components x 0 and y 0 increase from right to left, and | Ω Ω ⟩ ⟩ is the ground state (vacuum state) of the interacting theory, different from the free ground state | 0 ⟩ ⟩ . This expression represents the probability amplitude for the field to propagate from y to x, and goes by multiple names, like the two-point propagator, two-point correlation function, two-point Green's function or two-point function for short.