The work summarizes and discusses the basic conditions and rules for the use of the deductive method of syntaxonomic classification. This method develops and supplements classification procedures of the Braun-Blanquet approach. It enables to classify the communities which cannot be characterized by their own character- and differential-species. The deductive method incorporates into the inductively constructed system of abstract units of the Braun-Blanquet approach (classes, orders, alliances and associations) really existing communities on the basis of the presence or absence of diagnostic characters (character-and differential-species) of these abstract units. Some communities are incorporable only at the class level, others at the level of lower units. To objectify the process of incorporation of the communities represented by the respective phytocoenological releves, an algorithm of the deductive method was constructed which uniquely incorporates individual releves and groups of releves into the phytocoenological system on the basis of the presence, coverage and diagnostic value of species. The work includes a detailed description of this algorithm and examples of its application on concrete phytocoenological material. Examples of hitherto use of the deductive method are also discussed and its nomenclatural rules briefly explained.