  Slim semimodular lattices (for short, SPS lattices) and slim rectangular
lattices (for short, SR lattices) were introduced by G. Gr\"atzer and E. Knapp
in 2007 and 2009. These lattices are necessarily finite and planar, and they
have been studied in more then four dozen papers since 2007. They are best
understood with the help of their $\mathcal C_1$-diagrams, introduced by the
author in 2017. For a diagram $F$ of a finite lattice $L$ and a congruence
$\alpha$ of $L$, we define the ``quotient diagram'' $F/\alpha$ by taking the
maximal elements of the $\alpha$-blocks and preserving their geometric
positions. While $F/\alpha$ is not even a Hasse diagram in general, we prove
that whenever $L$ is an SR lattice and $F$ is a $\mathcal C_1$-diagram of $L$,
then $F/\alpha$ is a $\mathcal C_1$-diagram of $L/\alpha$, which is an SR
lattice or a chain. The class of lattices isomorphic to the congruence lattices
of SPS lattices is closed under taking filters. We prove that this class is
closed under two more constructions, which are inverses of taking filters in
some sense; one of the two respective proofs relies on an inverse of the
quotient diagram construction.
