<p>Nylon-6 exhibits several forms of crystal modification. The α form converts to the iodine complex when it is immersed in a highly concentrated KI/I₂ solution. After deiodization in a hypo (sodium thiosulfate) solution, the iodine complex changes to the γ form. The phase transition mechanism from the α to the γ form through the iodine complex has remained a challenging issue because of the lack of established crystal structures. As previously reported (<i>Polymer Journal</i>, 2025), a quantitative analysis of 2D wide-angle X-ray and neutron diffraction data necessitated a revision of the crystal structure of the α form by incorporating up/down chain packing disorder. This paper reports that a similar up/down chain disorder is also present in the crystal lattices of the γ form and the iodine complexes. In both the α and γ forms, the crystal lattice is composed of stacked hydrogen-bonded sheet planes. The aforementioned up/down chain packing disorder can be expressed in terms of the stacking disorder of sheets or the disordered slippages of sheets along the <i>a</i> axis. Thus, the notion of stacking disorder of sheet planes allows for a systematic and logical interpretation of the transition behaviors among these crystalline forms of nylon-6.</p>