The present article is concerned with how nanoparticles join: it considers MgO nano-cubes as a model, well-defined system. The development of grain boundaries (GBs) between cube particles has been re-examined using MgO smoke. In addition to the face-to-face contact which leads to the well-known low-a twist GBs, interactions are also found which initially involve point-to-face contact, edge-to-face contact, or contacts along the cube edges. It is proposed here that the point contact lead to a line contact through the requirement to balance charges, and rotation about such a line of contact leads to formation of the interface, i.e., the grain boundary. The atoms along the edges have lower coordination than the atoms in the bulk, which may contribute to the edge-edge and edge-face boundary formation. The inherently small size of nanoparticles makes transmission electron microscopy (TEM) an invaluable technique for characterizing the contacts between them without modifying them in any way. The present study uses TEM to characterize the types of boundaries formed, discusses the boundary structures, and considers how the particle morphology may determine the formation of low-a GBs.