The subject of this paper is research and mathematical analysis of the phenomenon of the disappearance of slow-growing faces. Results of this mathematical analysis show that some faces, for which a given geometrical condition is fulfilled, may disappear if they are growing more slowly than one of the neighbouring faces. For other faces there is no such possibility. It is shown that the crystal habit is determined not only by the growth conditions which influence the relative growth rates, but also, in great degree, by the geometry of the crystal.