Skinner's elementary verbal relations: Some new categories

In Verbal Behavior (1957) B. F. Skinner identified and named five elementary verbal relations: mand, tact, intraverbal, textual and echoic. Because of their etymological commitment to visual and auditory stimuli respectively, the last two categories do not function well as general categories. Adding two more general categories, codic and duplic, to the first three results in a set of five mutually exclusive and collectively exhaustive groupings. Textual behavior and other relations involving point-to-point correspondence but no formal similarity fall into the codic category. Echoic behavior and other relations with formal similarity fall into the duplic category. This arrangement results in useful category names for all elementary forms and prevents potentially confusing extensions, such as referring to Braille reading as textual behavior, or sign imitation as echoic behavior.