Poly(vinylcyclohexane) (PVCH) is a patented polymeric nucleating agent for isotactic polypropylene (iPP) and induces its alpha crystal modification (alpha iPP). The structural interactions at the root of this activity are investigated by analyzing the transcrystallization of single crystals of aiPP on single crystals of PVCH. Electron microscopy results indicate (i) a quasi-perfect two-dimensional lattice match in the contact plane, with (100)PVCH facimg (110)(alpha ipp) or (110)(alpha ipp) planes, and (ii) a consistent link between the PVCH single crystals' internal structure and the orientation of alpha iPP overgrowth. The two polymers chain axes are parallel. The 3-fold (iPP) and 4-fold (PVCH) helices share the same chain axis repeat (approximate to 6.5 angstrom), and repeat distances normal to the chain axis are approximate to 21.9 angstrom for (110)(alpha ipp) and (100)(PVCH) planes. In chain axis projection, the PVCH helices are tilted relative to the a and b axes of the tetragonal unit cell. The tilt generates an "asymmetric sawteeth" surface profile that differs for clock- or counterclockwise tilts. Similar surface profiles exist in the (110)(alpha ipp) or (110)(alpha ipp) contact planes. Matching of the PVCH and iPP sawteeth profiles is selective and induces a single orientation of the alpha iPP crystal lattice in the overgrowth. The orientation of the alpha iPP overgrowth thus becomes also a marker of the local clock- or counterclockwise setting of PVCH helices in the growth face.