By the application of the simple crystallographic operations translation, rotation and reflection to simple prototype structures modulated (super‐) structures may be derived. Irregular application of any operation leads to defect structures, which are best described as intergrowths of their different, ordered counterparts. The defects are of the ’’extended’’ type; ’’point’’ defects are unnecessary. In addition to modulation metal oxides other inorganic compounds, such as sulphides and halides, often exhibit the same or systems. Special consideration is given to the iron carbides, Fe3C and Fe5C2.