We have synthesized, by the active esters method, several polyamides (nylons) n,3 with n = 4-6, 8, 9, and 12. We have studied these polymers and another group of similar polymers obtained from carbon suboxide. The methods used involve IR and NMR spectroscopies, calorimetry, electron diffraction and microscopy, and X-ray diffraction. These polymers show structural polymorphism, in the sense that powder samples prepared directly after polymer synthesis have a poorly ordered structure that is different from that found in single crystals and oriented films. Single crystals have been obtained by solution crystallization; they usually have irregular shapes and borders. Electron diffraction of single crystals shows that the molecular chains pack in a quasi-hexagonal rectangular cell. Electron and X-ray diffraction of oriented films shows that the molecules pack in either an orthorhombic or a monoclinic unit cell, depending on the parity of n. The molecules are linked by a unique network of hydrogen bonds, with two spatial orientations that form an angle of about 120 degrees. Such an organization is quite different from the usual alpha- and gamma-forms of nylons.