AbstractAluminium plates with large crystals, obtained by recrystallization of finely grained material, show many “pointed” crystals: cf. fig. 1, 9 and 11. Such crystals have grown as a result of “stimulation” by another crystal, in this way that their growth did not start until another crystal, the growth of which had started previously, reached the nucleus of the “stimulated” crystal. In order to obtain a measurable size, the velocity of growth of the stimulated crystal must, at least at the beginning, be larger than that of the stimulating crystal.X‐ray investigation has shown that the relation between the orientations of both crystals is practically always that of a spinel‐twin: cf. fig. 5. Twinning is thus rather common in recrystallized aluminium. It has only not be generally recognized on account of the absence of straight twin boundaries, a consequence of the difference in velocity of growth of stimulating and stimulated crystal.The question is discussed as to the physical aspect of “stimulation” in the above given sense. For the moment the following has been suggested: Stimulating and stimulated crystal have an octahedral plane (111) and in this plane the direction [110] in common. This plane and direction are those used in the glide‐process of the cubic face‐centred metals. Suppose gliding along a definite combination has taken place in a lattice‐region: this means, according to the now prevalent conception, that “dislocations” have moved along this plane over certain distances and are then arrested, together with their fields of stress, at definite structural “irregularities” (flaws etc.). Suppose again there is a growing crystal, which by chance has an octahedral plane in twin orientation with regard to the active glide‐plane of the lattice‐region considered above. Then as soon as contact is established between the growing crystal and the deformed lattice‐region, something like a “discharge” of the arrested dislocations along the octahedral plane of the growing crystal might perhaps be conceived. As a consequence the deformed region may attain the capacity to grow. Why, however, its velocity of growth is larger than that of the stimulating crystal, is not yet clear. Perhaps the lattice of the stimulated crystal is more “perfect” than that of the stimulating crystal.The possibility must be kept in mind that the process described by us as a stimulation does not exist in reality and is actually caused by an “error” in the deposition of a layer of atoms along the octahedral plane, giving rise to the occurrence of the twin orientation. In that case it seems still more difficult to imagine in which way the experimentally observed difference in velocity of growth of both twin‐parts comes into existence. Perhaps this second mechanism is the normal one for such metals as nickel, copper, nickeliron, which twin frequen