This system is designed to provide simple descriptions of reaction mechanisms suitable for use in oral and written communication. The need arises because the system originally devised by Ingold is becoming increasingly overburdened with nonsystematic modifications. The new system describes a mechanism in terms of bond making (symbolized A for “attachment”) and breaking (D, “detachment”), with subscripts and other symbols to indicate the pattern of electron movement. For example, ANDN describes a single concerted process involving nucleophilic bond making and nucleofugic bond breaking (such as the SN2 Ingold mechanism), and AN + DN represents a two-step process (such as SN1). There is provision for describing homolytic and cyclic mechanisms, for diffusional control, and for electron transfer. Separate provision is made for symbolizing extra-mechanistic information, including the class of transformation, the nature of the substrate, and the occurrence of catalysis, using easily pronounced terms. Thus the symbol Em-AL-B describes an elimination from an alkyl substrate under general base catalysis .