The state or process wherein one crystalline material builds up a layer (usually thin) of its own crystal lattice upon the surface of some other crystal with a lattice geometry similar in crystal symmetry and lattice spacing to its own. A close match of lattice geometry gives rise to ice nucleation, at only a few degrees below 0°C, whereas a small water drop may normally supercool some tens of degrees K. The effect has been demonstrated for water vapor growth to ice crystals and freezing of supercooled droplets on large single crystals, oriented with respect to the base common occurrence crystal such as silver iodide. Molecule size and structure also influence the effect.