Original crystal structures of even-even polyamides are introduced. They are based on twisted conformations at the amide-aliphatic part links and are reminiscent of the pleated and rippled protein sheet structures introduced by Pauling and Corey in the early 1950s. Pleated sheets are chiral, and rippled sheets are made of two conformational isomers. They have specific X-ray diffraction "profiles". Publications on even-even polyamides describe such profiles, indicating that both pleated and rippled sheet variants exist. The pleated sheet structure is the stable crystal form of the whole family of nylons 2,2n. The rippled sheet applies for a high-temperature "alpha phase variant" of nylons with long aliphatic parts and more generally appears to be (one of?) the so-called "high-temperature a phase(s)" of even-even nylons. Pleated and rippled sheet crystal structures appear to be the entropically favored, high-temperature crystal form(s) of nylons. This new family of crystal structures and their possible variants with different sequences of twisted amide-aliphatic part junctions open a possible way for the (re)analysis of rare and unusual crystal morphologies produced at high Tc after partial melting/self-seeding.