A crystal form is a group of faces, all of which have the same relation to the functions of symmetry on the crystal — the symmetry on a crystal has a direct influence on which specific forms are present.
There are 48 possible crystal forms (47 forms as defined by crystallographers of the Fedorov Institute, see annotation) that can be developed as the result of the 32 combinations of symmetry. The forms present on crystals may be classified as (i) non-isometric and (ii) isometric — isometric crystals have essentially a different set of forms than the same in the other crystal systems.
The faces of a crystal form may intersect faces of another form on a crystal resulting in shapes being altered not resembling the perfect texbook full form shown above.