  IIn a finite lattice, a congruence spreads from a prime interval to another
by a sequence of congruence-perspectivities through \emph{intervals of
arbitrary size}, by a 1955 result of J. Jakub\'ik.
  In this note, I introduce the concept of \emph{prime-perspectivity} and prove
the Prime-projectivity Lemma: a congruence spreads from a prime interval to
another by a sequence of prime-perspectivities through \emph{prime ntervals}.
  A planar semimodular lattice is \emph{slim} if it contains no $\mathsf{M}_3$
sublattice. I introduce the Swing Lemma, a very strong version of the
Prime-projectivity Lemma for slim, planar, semimodular lattices.
