The article presents a comprehensive structural model for amyloid fibrils formed by the 40-residue β-amyloid peptide (Aβ1-40) associated with Alzheimer's disease. The model is based on solid-state NMR and electron microscopy data and specifically applies to fibrils with a periodically twisted morphology, characterized by a twist period of 120 ± 20 nm. The structure exhibits threefold symmetry about the fibril growth axis, as evidenced by mass-per-length data and 13C NMR signals. Comparison with previously reported models for Aβ1-40 fibrils with a striated ribbon morphology reveals differences in overall symmetry, conformation of non-β-strand segments, and quaternary contacts. Both morphologies contain in-register parallel β-sheets, constructed from nearly the same β-strand segments. The study highlights the molecular basis for the polymorphism observed in Aβ1-40 fibrils and suggests that these structural variations may have broader implications for amyloid fibril formation in other diseases and biological functions.The article presents a comprehensive structural model for amyloid fibrils formed by the 40-residue β-amyloid peptide (Aβ1-40) associated with Alzheimer's disease. The model is based on solid-state NMR and electron microscopy data and specifically applies to fibrils with a periodically twisted morphology, characterized by a twist period of 120 ± 20 nm. The structure exhibits threefold symmetry about the fibril growth axis, as evidenced by mass-per-length data and 13C NMR signals. Comparison with previously reported models for Aβ1-40 fibrils with a striated ribbon morphology reveals differences in overall symmetry, conformation of non-β-strand segments, and quaternary contacts. Both morphologies contain in-register parallel β-sheets, constructed from nearly the same β-strand segments. The study highlights the molecular basis for the polymorphism observed in Aβ1-40 fibrils and suggests that these structural variations may have broader implications for amyloid fibril formation in other diseases and biological functions.