Set and
. The diagram then has the following form:

When we pass to the derived couple, each group is replaced by a subgroup
. The differentials
go two units to the right, and we replace the term
by the term
, where the
‘s refer to the
‘s leaving and entering
respectively.

The maps now go diagonally upward because of the formula
. The maps
and
still go vertically and horizontally,
being a restriction of
and
being induced by
.