In a spin system, the equal-time correlation function is especially well-studied. It describes the canonical ensemble (thermal) average of the scalar product of the spins at two lattice points over all possible orderings: C ( r ) = ⟨ ⟨ s ( R ) ⋅ ⋅ s ( R + r ) ⟩ ⟩ − − ⟨ ⟨ s ( R ) ⟩ ⟩ ⟨ ⟨ s ( R + r ) ⟩ ⟩ . Here the brackets mean the above-mentioned thermal average. Schematic plots of this function are shown for a ferromagnetic material below, at, and above its Curie temperature on the left.