Nanocrystal shape can have a strong influence on the 2D and 3D structure of nano(NC) arrays, and it is important to its role if functional magnetic nanodevices are to be realized. Here is presented a detailed TEM analysis of 11 nm Co NCs and their effect on the superlattice assembly. Twins and stacking faults (see Figure) are reported to be due to mismatched faces on adjacent nanocrystals (see also cover).