  Opposite-subsystem twin events and twin observables, studied previously in
the context of distant correlations, are first generalized to pure states of
not-necessarily-composite systems, and afterwards they are further generalized
to delayed twins that are due to unitary evolution of the quantum system. The
versatile aspects of delayed twin observables are studied in terms of necessary
and sufficient conditions to make possible various applications. Three of these
are sketched: Preparation of some quantum experiments, easy solution of a
puzzle in an important Scully et al. real experiment, and, finally, it is shown
that exact measurement in quantum mechanics is an example of opposite-subsystem
delayed twins in bipartite pure states.
