  We have observed boundaries between hcp $^4$He crystal grains in equilibrium
with liquid $^4$He. We have found that, when emerging at the liquid-solid
interface, a grain boundary makes a groove whose dihedral angle $2\theta$ is
non-zero. This measurement shows that grain boundaries are not completely wet
by the liquid phase, in agreement with recent Monte Carlo simulations.
Depending on the value of $\theta$, the contact line of a grain boundary with a
solid wall may be wet by the liquid. In this case, the line is a thin channel
with a curved triangular cross section, whose measured width agrees with
predictions from a simple model. We discuss these measurements in the context
of grain boundary premelting and for a future understanding of the possible
supersolidity of solid $^4$He.
