  The combined analysis of $\nu_\mu$ disappearance and $\nu_e$ appearance data
of NO$\nu$A experiment leads to three nearly degenerate solutions. This
degeneracy can be understood in terms of deviations in $\nu_e$ appearance
signal, caused by unknown effects, with respect to the signal expected for a
reference set of oscillations parameters. We define the reference set to be
vacuum oscillations in the limit of maximal $\theta_{23}$ and no CP-violation.
We then calculate the deviations induced in the $\nu_e$ appearance signal event
rate by three unknown effects: (a) matter effects, due to normal or inverted
hierarchy (b) octant effects, due to $\theta_{23}$ being in higher or lower
octant and (c) CP-violation, whether $\delta_{CP} \sim - \pi/2$ or $\delta_{CP}
\sim \pi/2$. We find that the deviation caused by each of these effects is the
same for NO$\nu$A. The observed number of $\nu_e$ events in NO$\nu$A is
equivalent to the increase caused by one of the effects. Therefore, the
observed number of $\nu_e$ appearance events of NO$\nu$A is the net result of
the increase caused by two of the unknown effects and the decrease caused by
the third. Thus we get the three degenerate solutions. We also find that
further data by NO$\nu$A can not distinguish between these degenerate solutions
but addition of one year of neutrino run of DUNE can make a distinction between
all three solutions. The distinction between the two NH solutions and the IH
solution becomes possible because of the larger matter effect in DUNE. The
distinction between the two NH solutions with different octants is a result of
the synergy between the anti-neutrino data of NO$\nu$A and the neutrino data of
DUNE.
