The crystal structures of 1,4-dibenzoylbutane, 2DBC, and ethyleneglycol dithiobenzoate, 2DBS, have been solved by direct methods in order to follow the evolution of the geometry and the conformation of the Y—CH2—CH2—Y sequence in the isoelectronic series of C6H5—CO—Y—CH2—CH2—Y—CO—C6H5 where Y = CH2, NH, O, and S. 2DBC is monoclinic, a = 7.750(4), b = 21.287(6), c = 8.905(2) Å, β = 100.55(3)°, Z = 4. 2DBS is also monoclinic, with a = 5.702(3), b = 16.171(15), c = 8.001(4) Å, β = 98.79(4)°, and Z = 2. Both compounds belong to the P21/n space group. The weighted least-squares refinements converged to Rw = 0.049 (2DBC) and 0.043 (2DBS). The conformation of the methylenic sequence is all-trans (2DBC) and g+tg− for 2DBS (t = trans, g = gauche). The ir spectra have absorptions at 1467, 1308, and 931 cm−1 (2DBC) and at 1408, 1239, and 727 cm−1 (2DBS), respectively, characteristic of the trans conformation of the Y—CH2—CH2—Y sequence. The 13C CP/MAS solid-state nmr spectra of these and other related compounds have been recorded. It is, in turn, possible to propose that the τ torsion angle between the amide group and the methylenic sequence, has a value of 85° in the aromatic polyamide, Nylon 2T.