Other less popular proposed solutions exist such as Nelson–Barr models. These set θ θ ¯ ¯ = 0 at some high energy scale where CP-symmetry is exact but the symmetry is then spontaneously broken at low energies. The tricky part of these models is to account for why θ θ ¯ ¯ remains small at low energies while the CP breaking phase in the CKM matrix becomes large.