  In an earlier paper, to describe how a congruence spreads from a prime
interval to another in a finite lattice, I introduced the concept of
prime-perspectivity and its transitive extension, prime-projectivity and proved
the Prime-projectivity Lemma. In this paper, I specialize the
Prime-projectivity Lemma to slim, planar, semimodular lattices to obtain the
Swing Lemma, a very powerful description of the congruence generated by a prime
interval in this special class of lattices.
