Over a period spanning more than twenty years (1984–2007), Wertsch has provided examples of learning–teaching situations to illustrate various theoretical concepts he has applied to explicate and elaborate Vygotsky's concept of mediation and the zone of proximal development. The examples include a puzzle copying task (1979, 1984, 1985), an instance of reciprocal learning (1998), and the learning of elementary statistical concepts by means of graphic representation (2007). In his early work with the puzzle task, in which he focused on the zone of proximal development, Wertsch introduced the ‘explanatory concepts’ of situation definition, negotiation, intersubjectivity and semiotic mediation (1984, 1985), drawing on the work of Rommetveit (1979). He commented then that he ‘focused on these theoretical constructs because they seem to be points over which confusion is most likely to arise’ and also that ‘they are issues that are either presupposed or explicitly involved in much of the existing research on the zone of proximal development’ (1984, p. 16). With the example of reciprocal teaching, Wertsch added to the mix his own concepts of mediational means and cultural tools as well as the concept of the materiality of mediational means, all of which he developed in his account of Mind in Action (1998). In the last example of the learning and teaching of statistical concepts, Wertsch (2007) supplements his theoretical arsenal with the Vygotskian principle that word meaning develops.