  We introduce the representation on one unitarity triangle of the constraints
resulting (1) from the interpretation of solar and atmospheric neutrino data in
terms of oscillations, and (2) from the search for neutrinoless double beta
decay. We show its use for the study of a nearly degenerate neutrino spectrum.
The representation shows clearly the particular cases when the neutrinoless
double beta decay rate can (or cannot) be small, that is: when the connection
of the decay rate with the neutrino spectrum is less (or more) direct. These
cases turn out to depend crucially on the scenario of oscillation (MSW
solutions, vacuum oscillations, averaged oscillations), and in particular on
the size of the mixing between the electron neutrino and the neutrino state
giving rise to atmospheric neutrino oscillations.
