  Nematic twist--bend is the fifth nematic phase recognized in nature. This
phase exhibits spiral orientational order, thus it is a chiral structure and it
can be stabilized in systems composed of achiral molecules. The microscopic
origin of this spontaneous chiral symmetry breaking is to a large extent
unknown but phenomenologically it appears stabilized by assuming coupling
between steric polar and orientational orderings. Understanding of how external
fields affect the stability of this phase is of great intellectual interest and
of relevance to potential applications. Within mesoscopic Landau--de Gennes
theory we find that for compounds with positive anisotropy the helix unwinds to
a polar uniaxial nematic, however negative material anisotropy gives rise to a
rich sequence of new nematic phases obtained via mechanism of flattening the
conical spiral.
