In order to carry out its project for a mechanized documentation service based on a system of keyword coordination, European Atomic Energy Community's Centre for Information and Documentation was led to draw up an inventory of terminology in the nuclear field and maintain a tight control over it. A basic thesaurus of 1230 keywords was drawn up and 3240 other keywords was drawn up and 3240 other keywords can be generaged by two simple combinatory rules. The keywords are arranged in arrowgraphs which facilitate considerably the indexing of both documents and queries. Specific terms not contained in the thesaurus can be used in addition to the keywords in order to make the indexing more accurate. The control and standardization of these "additional" terms are partly automatic, these operations being carried out with the aid of a "Dictionary" consisting of the keywords, the standardized additional terms and the forbidden terms. The dictionary is represented by terminology charts derived from the arrowgraphs. (Author)