  The hexagonal RMnO$_3$ (h-RMnO$_3$) are multiferroic materials, which exhibit
the coexistence of a magnetic order and ferroelectricity. Their distinction is
in their geometry that both results in an unusual mechanism to break inversion
symmetry, and also produces a 2D triangular lattice of Mn spins, which is
subject to geometrical magnetic frustration due to the antiferromagnetic
interactions between nearest neighbour Mn ions. This unique combination makes
the h-RMnO$_3$ a model system to test ideas of spin-lattice coupling,
particularly when the both the improper ferroelectricity and the Mn
trimerisation that appears to determine the symmetry of the magnetic structure
arise from the same structure distortion. In this review, we demonstrate how
the use of both neutron and X-ray diffraction and inelastic neutron scattering
techniques have been essential to paint this comprehensive and coherent picture
of the h-RMnO$_3$.
