  More than 70 years ago, Jaques Riguet suggested the existence of an
``analogie frappante'' (striking analogy) between so-called ``relations de
Ferrers'' and a class of difunctional relations, members of which we call
``diagonals''. Inspired by his suggestion, we formulate an ``analogie
frappante'' linking the notion of a block-ordered relation and the notion of
the diagonal of a relation. We formulate several novel properties of the
core/index of a diagonal, and use these properties to rephrase our ``analogie
frappante''. Loosely speaking, we show that a block-ordered relation is a
provisional ordering up to isomorphism and reduction to its core. (Our theorems
make this informal statement precise.) Unlike Riguet (and others who follow his
example), we avoid almost entirely the use of nested complements to express and
reason about properties of these notions: we use factors (aka residuals)
instead. The only (and inevitable) exception to this is to show that our
definition of a ``staircase'' relation is equivalent to Riguet's definition of
a ``relation de Ferrers''. Our ``analogie frappante'' also makes it obvious
that a ``staircase'' relation is not necessarily block-ordered, in spite of the
mental picture of such a relation presented by Riguet.
