  The coupling of magnetic chiralities to the ferroelectric polarisation in
multiferroic RbFe(MoO$_4$)$_2$ is investigated by neutron spherical
polarimetry. Because of the axiality of the crystal structure below
$T_\textrm{c}$ = 190 K, helicity and triangular chirality are
symmetric-exchange coupled, explaining the onset of the ferroelectricity in
this proper-screw magnetic structure - a mechanism that can be generalised to
other systems with "ferroaxial" distortions in the crystal structure. With an
applied electric field we demonstrate control of the chiralities in both
structural domains simultaneously.
