In 1995 I introduced the idea of intervening duality with the context of matching pennies games between two persons. I subsequently extended that idea with papers relating first to experimenter-experiment interactions via an intervening — and explicitly specified — die casting experiment and, secondly, to farmer-landowner rent bargaining with intervening weather forecasts modelled as bargaining instruments. The purpose of the present paper is to provide general results which not only specialise to these three previous classes of applications, but which use the idea of strategic equivalence to provide a formal basis for the analysis of large classes of explicitly nonconstant sum bimatrix games, including the prisoners dilemma.