  In this paper we study the directions of periodicity of three-dimensional
subshifts of finite type (SFTs) and in particular their slopes. A configuration
of a subshift has a slope of periodicity if it is periodic in exactly one
direction, the slope being the angle of the periodicity vectors. In this paper,
we prove that any $\Sigma^0_2$ set may be realized as a a set of slopes of an
SFT.
