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High-resolution (0.2 nm) images are used to locate chemical domains occurring with length scales of 1–5 nm in the dielectric relaxor lead magnesium niobate (PMN). The experimental HRTEM images are analysed using computer simulations and image matching in order to clarify and characterize the nature of the chemical ordering. Madelung electrostatic energy calculations are used to rank a set of structural models for possible ordered and disordered distributions of Nb and Mg over the B sites of perovskite ABO3. Next, the chemical domain textures are modelled using next-nearest-neighbour Ising (NNNI) models and Monte Carlo methods. These simulations allow us to understand and quantify the local structures of clusters of Nb and Mg atoms and how these interact to form chemical domain walls and other configurations including charged chemical defects. This results in a preferred model for the B-site distribution (the extended NNNI model), which is used for image simulations. Both HRTEM many-beam bright- and dark-field and single-beam dark-field TEM images are obtained and compared with the experimental images. The final result is a realistic atomic model for the Nb, Mg distribution of PMN.