Using a Monte Carlo method, the measurement by the 1V1E technique of the average prompt neutron multiplicity as a function of the mass of fragments from the thermal neutron-induced fission of 239Pu is simulated. The input data, associated with the masses of complementary primary fragments (A1, A2), consist of the yield (Y), the average total kinetic energy (TKE-) and its standard deviation (σTKE), the average prompt neutron multiplicity (ν-s, a sawtooth approach of an experimental curve ν-), and the slope of neutron multiplicity against total kinetic energy, -dν/d<TKE>, plotted as a function of fragment mass. The output data, associated with the provisional masses of complementary fragments (m1∗, m2∗) calculated with the 1V1E method, consist of the simulated quantities Y, TKE-, σTKE, N-, comparable to the measured ones. In comparison with ν-s, N- and ν-are oversized in the region of light fragment masses near the symmetric fission. To interpret the results, the neighboring masses ratio RA =YA+1/Y(A) and the term FA=A-m∗, where m∗ is the average of provisional mass as a function of A, are defined. It is shown that N–ν-s and ν–ν-s are correlated with -F(R-1). It is concluded that the oversize of ν- relative to ν-s is due to the interplay of the prompt neutron emission and the slope of the mass yield curve.