 Such a ‘measurement-like’ interaction intuitively does not disturb the eigenstates of the monitored observable. Thus these preferred states can in fact be characterised in terms of their robustness or stability with respect to the interaction with the environment. The system gets entangled with the environment, but the states between which interference is suppressed are the ones that would themselves get least entangled with the environment under this interaction. In this connection, one also says that decoherence induces ‘effective superselection rules’, meaning the following. A strict superselection rule applies when there are some observables – in technical terminology they are called classical – that commute with all observables (for a review, see Wightman 1995). Intuitively, these observables are infinitely robust, since no possible interaction can disturb them (at least as long as the interaction Hamiltonian is considered to be an observable). By an effective superselection rule one means, analogously, that certain observables (e.g. chirality) will not be disturbed by the interactions that actually take place.