  In this paper, we discuss centroaffine geometry of polygons in $3$-space. For
a polygon $X$ that is locally convex with respect to an origin together with a
transversal vector field $U$, we define the centroaffine dual pair $(Y,V)$
similarly to [6]. We prove that vertices of $(X,U)$ correspond to flattening
points for $(Y,V)$ and also that constant curvature polygons are dual to planar
polygons. As an application, we give a new proof of a known $4$ flattening
points theorem for spatial polygons.
