Properties of the organic matrix of bone¹ as well as its function in the microstructure² could be the key to the remarkable mechanical properties of bone³. Previously, it was found that on the molecular level, calcium-mediated sacrificial bonds increased stiffness and enhanced energy dissipation in bone constituent molecules^4,5. Here we present evidence for how this sacrificial bond and hidden length mechanism contributes to the mechanical properties of the bone composite, by investigating the nanoscale arrangement of the bone constituents^6–8 and their interactions. We find evidence that bone consists of mineralized collagen fibrils and a non-fibrillar organic matrix², which acts as a ‘glue’ that holds the mineralized fibrils together. We believe that this glue may resist the separation of mineralized collagen fibrils. As in the case of the sacrificial bonds in single molecules⁵, the effectiveness of this mechanism increases with the presence of Ca²⁺ ions.