  The classical involutive division theory by Janet decomposes in the same way
both the ideal and the escalier. The aim of this paper, following Janet's
approach, is to discuss the combinatorial properties of involutive divisions,
when defined on the set of all terms in a fixed degree D, postponing the
discussion of ideal membership and related test. We adapt the theory by Gerdt
and Blinkov, introducing relative involutive divisions and then, given a
complete description of the combinatorial structure of a relative involutive
division, we turn our attention to the problem of membership. In order to deal
with this problem, we introduce two graphs as tools, one is strictly related to
Seiler's L-graph, whereas the second generalizes it, to cover the case of
"non-continuous" (in the sense of Gerdt-Blinkov) relative involutive divisions.
Indeed, given an element in the ideal (resp. escalier), walking backwards
(resp. forward) in the graph, we can identify all the other generators of the
ideal (resp. elements of degree D in the escalier).
