Models are presented for the family of n,3 polyamides. They have an organization in space quite different from the usual alpha and gamma forms of polyamides. Hydrogen-bonded sheets are not present in the n,3 polyamides. The molecules are linked by an unique network of hydrogen bonds with two spatial orientations which form an angle of about 120 degrees. The different organization of the molecules when n = odd (orthorhombic) and n = even (monoclinic) is shown to be due to the optimization of the hydrogen-bond geometry. The polymethylene segments deviate somewhat from an all-trans conformation.