Phenomenology of crystal polymorphism

For DCLANT (9,10-dichloroanthracene) one sees three determinations in PI, in all evidence corresponding to the same polymorph, although cell parameters are different because the authors did not care to use the same reduced cell. Practicing crystallographers tend to consider the standardization of cell setting as a minor whim, while it could save a great deal of effort of theoretical chemists and solid state chemists 

A fairly common occurrence, which could be called semi-polymorphism, is that of structures that are identical except for the doubling of one cell parameter, changing from a given space group into a subgroup with loss of one symmetry element (Table 14.2). Another similar circumstance is the formal loss of a symmetry element with a doubling of the number of molecules in the asymmetric unit, which, more often than not, are almost exactly related by that symmetry operation. The diffracted 

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