These rules provide a general system of nomenclature for transformations whereby one organic compound is converted into another. A transformation is distinct from a reaction in that it describes only those changes that are involved in converting the structure of a substrate into that of a product, regardless of the reagent or the precise nature of the substrate, or (with some exceptions) the mechanism. Thus all processes in which X-H is converted into X-NO₂ are examples of the single transformation called “nitration”, whatever the nature of X, and irrespective of whether the reaction entails the replacement of H⁺ by N0₂⁺, of H' by NO₂', or of H⁻ by N0₂⁻. The basis of the names of all transformations is that they provide a description of the conversion of substrate into product by giving a string or strings of the names of groups or entities that become attached to and/or detached from the substrate, followed by a suffix that describes the nature of the transformation. Straightforward examples are self-explanatory; for example: “hydro,chloro-addition”; “dibromo-elimination”; “hydroxy-de-iodo-substitution”. For many transformations, particularly substitutions, simplified names are recommended for use in speech or writing (by contrast to the more detailed indexing names). Thus “hydroxy-de- iodination” may be used instead of “hydroxy-de-iodo-substitution”, or “nitration” instead of “nitro-de-hydro-substitution”. A list of non-systematic names is given for transformations that are too complex to be named by the present systematic rules.