  ReS$_2$ has recently emerged as a new member in the rapidly growing family of
two-dimensional materials. Unlike MoS$_2$ or WSe$_2$, the optical and
electrical properties of ReS$_2$ are not isotropic due to the reduced symmetry
of the crystal. Here, we present layer-dependent Raman measurements of ReS$_2$
samples ranging from monolayers to ten layers in the ultralow frequency regime.
We observe layer breathing and shear modes which allow for easy assignment of
the number of layers. Polarization-dependent measurements give further insight
into the crystal structure and reveal an energetic shift of the shear mode
which stems from the in-plane anisotropy of the shear modulus in this material.
