The crystalline structure of nylon 12 2 has been studied by means of X-ray and electron diffraction. Two structures can be obtained, depending on the preparation conditions. Both are based on a progressive stacking of hydrogen-bonded sheets, but they differ in the chain axis shift between consecutive sheets. Packing energy calculations corroborate these observations and indicate as the most stable a structure with only 1 Angstrom shift. In the same way, theoretical calculations were also performed with nylon 6 2. Similarly, a structure with a different shift from the characteristic value of most conventional nylons is stabilized. Electron charge distributions on the oxalamide and amide groups play a fundamental role, thus favoring different stacking modes.