The common law doctrines relating to mixtures and improvements of personal chattels give the impression, to most English lawyers, of being somewhat mysterious. Proprietary interests come and go like ships in the night, now mingled with rights to possession, now quite separate. But although cases involving proprietary claims are not common, the issue often arises as to A's personal rights against B, where B has without A's consent expended effort on A's property, and B's personal rights against A (if any) in respect of the improvement.