Defining grain boundaries: an historical perspective

Any attempt to summarise the concept of 'grain boundaries' in a short article is destined to fail, but by focusing on one aspect of the literature, the coincident site lattice model, it may be possible to provide an historical perspective that includes some insight. This contribution attempts to outline the symbiotic relation between the development of the tools of microstructural investigation in the twentieth century and the dawning understanding of the complexity of interface and interphase boundaries in engineering materials.