When a single crystal deforms by glide which is unevenly distributed over the glide surfaces
the lattice becomes curved. The constant feature of distortion by glide on a single set of
planes is that the orthogonal trajectories of the deformed glide planes (the c-axes in
hexagonal metals) are straight lines. This leads to the conclusion that in polygonisation
experiments on single hexagonal metal crystals the polygon walls are planes, while the
glide planes are deformed into cylinders whose sections are the involutes of a single curve …