I'm trying to interpret graphs of this type. In this example, dispersion relation $\omega = \omega(k)$ for some waves corresponding to lattice vibrations in GaAs are shown.
What is uncomprehensible to me is the abscissa that should represent the 3D wave vector $\vec{k}$ in some way. Units in these graphs are never displayed. Some points along the axis are marked with letters that represent "high symmetry points" or "critical points" in Brillouin zone of the 3D lattice and therefore specific value for $\vec{k}$.
QUESTIONS:
a) How are the "high symmetry points" chosen? Do they have particular properties?
b) Which value of $\vec{k}$ (modulus and direction) does correpond to a point on the horizontal axis of the example graph that is found between two high symmetry points?