This study reports the atomic structure of the cross-β spine of amyloid-like fibrils formed by the yeast protein Sup35. The structure reveals a double β-sheet with parallel, in-register strands. Sidechains from the two sheets form a dry, tightly self-complementing steric zipper, bonding the sheets. Within each sheet, every segment is bound to its neighbors via stacks of both backbone and sidechain hydrogen bonds. The structure illuminates the stability of amyloid fibrils, their self-seeding characteristic, and their tendency to form polymorphic structures. The cross-β spine is a common structural feature of amyloid-like fibrils from different proteins. It consists of β-sheets parallel to the fibril axis, with strands perpendicular to this axis. The structure of GNNQQNY and related peptides reveals a pair-of-sheets organization with a dry interface composed of tightly interdigitated sidechains, forming a steric zipper. This interface is dry, in contrast to the highly hydrated external faces of the paired sheets. The dry interface has unusually high complementarity, as quantified by the S_C parameter. The structure of GNNQQNY shows limited similarity to β-helices proposed as models for amyloid and prion spines. The structure of GNNQQNY is similar to SufD, a member of the β-helix family, but differs significantly from cylindrical and triangular β-helices. The structure-based energetics suggest factors that determine the rate and stability of fibril formation, as well as a factor that may underlie amyloid fibril polymorphism and prion strains. The structure of GNNQQNY reveals that the self-complementary steric zipper explains how short segments of proteins can form amyloid-like fibrils. The structure also shows that the self-complementary GNNQQNY sequence is a segment of the yeast prion Sup35, which converts copies of itself to an amyloid fibril-like state. This fibrillar state is at the basis of the transition to the [PSI+] prion state of Sup35. The structure of GNNQQNY and NNQQNY suggests that a tight, dry steric fit between a pair of sheets is likely to be a fundamental feature of amyloid-like fibrils. However, it is not yet clear how to reconcile this with evidence from mass-per-unit-length measurements on Aβ fibrils and from EM measurements of GNNQQNY protofibrils, which are consistent with four sheets. The structure of GNNQQNY also reveals that amide stacks and tyrosine stacks contribute to the stability of amyloid-like fibrils. The structure-based energetics suggest that the formation of amyloid-like fibrils is influenced by the concentration of proteins and the presence of the steric zipper and hydrogen bond stacks. The study provides insights into the biological implications of the structure, including the regulation of protein concentration within cells and tissues to prevent fibrThis study reports the atomic structure of the cross-β spine of amyloid-like fibrils formed by the yeast protein Sup35. The structure reveals a double β-sheet with parallel, in-register strands. Sidechains from the two sheets form a dry, tightly self-complementing steric zipper, bonding the sheets. Within each sheet, every segment is bound to its neighbors via stacks of both backbone and sidechain hydrogen bonds. The structure illuminates the stability of amyloid fibrils, their self-seeding characteristic, and their tendency to form polymorphic structures. The cross-β spine is a common structural feature of amyloid-like fibrils from different proteins. It consists of β-sheets parallel to the fibril axis, with strands perpendicular to this axis. The structure of GNNQQNY and related peptides reveals a pair-of-sheets organization with a dry interface composed of tightly interdigitated sidechains, forming a steric zipper. This interface is dry, in contrast to the highly hydrated external faces of the paired sheets. The dry interface has unusually high complementarity, as quantified by the S_C parameter. The structure of GNNQQNY shows limited similarity to β-helices proposed as models for amyloid and prion spines. The structure of GNNQQNY is similar to SufD, a member of the β-helix family, but differs significantly from cylindrical and triangular β-helices. The structure-based energetics suggest factors that determine the rate and stability of fibril formation, as well as a factor that may underlie amyloid fibril polymorphism and prion strains. The structure of GNNQQNY reveals that the self-complementary steric zipper explains how short segments of proteins can form amyloid-like fibrils. The structure also shows that the self-complementary GNNQQNY sequence is a segment of the yeast prion Sup35, which converts copies of itself to an amyloid fibril-like state. This fibrillar state is at the basis of the transition to the [PSI+] prion state of Sup35. The structure of GNNQQNY and NNQQNY suggests that a tight, dry steric fit between a pair of sheets is likely to be a fundamental feature of amyloid-like fibrils. However, it is not yet clear how to reconcile this with evidence from mass-per-unit-length measurements on Aβ fibrils and from EM measurements of GNNQQNY protofibrils, which are consistent with four sheets. The structure of GNNQQNY also reveals that amide stacks and tyrosine stacks contribute to the stability of amyloid-like fibrils. The structure-based energetics suggest that the formation of amyloid-like fibrils is influenced by the concentration of proteins and the presence of the steric zipper and hydrogen bond stacks. The study provides insights into the biological implications of the structure, including the regulation of protein concentration within cells and tissues to prevent fibr