Page "Absolute zero" Paragraph 8

A perfect crystal is one in which the internal lattice structure extends uninterrupted in all directions.

The perfect order can be represented by translational symmetry along three ( not usually orthogonal ) axes.

Every lattice element of the structure is in its proper place, whether it is a single atom or a molecular grouping.

For substances which have two ( or more ) stable crystalline forms, such as diamond and graphite for carbon, there is a kind of " chemical degeneracy ".

The question remains whether both can have zero entropy at T = 0 even though each is perfectly ordered.