The number of planes is extremely variable in many crystals. Those of the same substance produced under modified conditions, even when issuing from the same solution, are occasionally very different in this respect. Generally the forms become less complicated and the number of planes diminishes, a certain number Of them definitely disappearing, during the crystal's increase in volume. Such transitory planes are very frequently defective in parallelism.

Two kinds of planes may be distinguished on crystals of any system ; some which are constant, others which may be termed variable; the latter being occasionally wanting, are in other cases very numerous.