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The accurate representation of geometrical relations in three dimensions is always a matter of difficulty, and this is especially the case with the forms of crystals.

The most obvious course is to resort to models, but these are troublesome to construct, and occupy considerable space, so that for all practical purposes some method of representation in two dimensions, in other words a projection on a plane surface, such as a sheet of paper, must be employed.<br><br>The stereographic, gnomonic and linear projections, though admirable as exact records of the disposition in space of the faces, are unsuited to the elementary student who requires a projection that will enable him to realise at once the general form of the crystal. Ordinary perspective is, on the other hand, objectionable because the parallelism of lines is not preserved, and the scale of representation varies according to the distance from the point of view.