The moiré effect in 3D objects with planar facets is considered. The projected period of the inclined periodic grating was found. The formula for the period of the moiré patterns in inclined plain surfaces was obtained for objects with arbitrary oriented plain facets, namely, the parallelepiped and the prism (parallel and non-parallel facets). The similarity between the projected period and the moiré period was demonstrated. The direction to the longest moiré pattern in the wedge was found theoretically and observed in experiments. The results can be used in the alignment of flat surfaces.