This study presents a detailed structural model of amyloid fibrils formed by the 40-residue β-amyloid peptide (Aβ1-40) associated with Alzheimer's disease. The model is based on experimental data from solid-state NMR and electron microscopy, and it describes 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 indicated by mass-per-length data and the observation of a single set of 13C NMR signals. The model reveals the molecular basis for polymorphism in Aβ1-40 fibrils, showing that different morphologies differ in overall symmetry, conformation of non-β-strand segments, and quaternary contacts, while sharing common secondary and tertiary structures. Both morphologies contain in-register parallel β-sheets, constructed from nearly the same β-strand segments. The findings suggest that structural variations in amyloid fibrils may have general implications for other polypeptides, such as amylin, which forms amyloid fibrils associated with type 2 diabetes. The study also provides insights into the structural and functional properties of amyloid fibrils, including the role of hydrophobic and polar interactions in stabilizing the fibril structure, and the potential for designing small molecules to target specific fibril morphologies. The results highlight the importance of structural diversity in amyloid fibrils and its implications for disease progression and therapeutic strategies.This study presents a detailed structural model of amyloid fibrils formed by the 40-residue β-amyloid peptide (Aβ1-40) associated with Alzheimer's disease. The model is based on experimental data from solid-state NMR and electron microscopy, and it describes 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 indicated by mass-per-length data and the observation of a single set of 13C NMR signals. The model reveals the molecular basis for polymorphism in Aβ1-40 fibrils, showing that different morphologies differ in overall symmetry, conformation of non-β-strand segments, and quaternary contacts, while sharing common secondary and tertiary structures. Both morphologies contain in-register parallel β-sheets, constructed from nearly the same β-strand segments. The findings suggest that structural variations in amyloid fibrils may have general implications for other polypeptides, such as amylin, which forms amyloid fibrils associated with type 2 diabetes. The study also provides insights into the structural and functional properties of amyloid fibrils, including the role of hydrophobic and polar interactions in stabilizing the fibril structure, and the potential for designing small molecules to target specific fibril morphologies. The results highlight the importance of structural diversity in amyloid fibrils and its implications for disease progression and therapeutic strategies.