  We analyze status of ${\bf C}$, ${\bf P}$ and ${\bf T}$ discrete symmetries
in application to neutron-antineutron transitions breaking conservation of
baryon charge ${\cal B}$ by two units. At the level of free particles all these
symmetries are preserved. This includes ${\bf P}$ reflection in spite of the
opposite internal parities usually ascribed to neutron and antineutron.
Explanation, which goes back to the 1937 papers by E. Majorana and by G. Racah,
is based on a definition of parity satisfying ${\bf P}^{2}=-1$, instead of
${\bf P}^{2}=1$, and ascribing $ {\bf P}=i$ to both, neutron and antineutron.
We apply this to ${\bf C}$, ${\bf P}$ and ${\bf T}$ classification of six-quark
operators with $|\Delta {\cal B} |=2$. It allows to specify operators
contributing to neutron-antineutron oscillations. Remaining operators
contribute to other $|\Delta {\cal B} |=2$ processes and, in particular, to
nuclei instability. We also show that presence of external magnetic field does
not induce any new operator mixing the neutron and antineutron provided that
rotational invariance is not broken.
