The assembly of proteins into fibrillar amyloid structures was once considered to be pathologic and essentially irreversible. Recent studies reveal amyloid-like structures that form reversibly, derived from protein low-complexity domains which function in cellular metabolism. Here, by comparing atomic-level structures of reversible and irreversible amyloid fibrils, we find that the β-sheets of reversible fibrils are enriched in flattened (as opposed to pleated) β-sheets formed by stacking of extended β-strands. Quantum mechanical calculations show that glycine residues favor extended β-strands which may be stabilized by intraresidue interactions between the amide proton and the carbonyl oxygen, known as C5 hydrogen-bonds. Larger residue side chains favor shorter strands and pleated sheets. These findings highlight a structural element that may regulate reversible amyloid assembly.