Conventionally, they are displayed as isosurfaces with positive density—electron density where theres nothing in the model, usually corresponding to some constituent of the crystal that hasnt been modelled, for example a ligand or a crystallisation adjutant -- in green, and negative density—parts of the model not backed up by electron density, indicating either that an atom has been disordered by radiation damage or that it is modelled in the wrong place—in red. The typical contouring (display threshold) is set at 3&sigma;.