Policy instruments are a fundamental component of public policies. Policy instruments are often a result of mediation within the policy design process, whenever decision makers reshape existing instruments without introducing any real innovation. This results in imitation, layering and ambiguity in tool choice selection, and raises the theoretical problem of the logic according to which decision makers choose certain specific policy instruments rather than others. Decision makers may have different reasons for choosing certain specific instruments, although these reasons should be connected to the two main purposes of decision-making, that is, the search for effectiveness and the construction of a shared sense, a common acceptance. Thus, the choice of instruments is a question of potentially conflicting drivers that decision makers have to cope with within a specific decisional situation, when asked to solve those problems that have arisen. This paper examines this question and offers an analytical framework based on the two main factors in terms of which the selection of instruments is channelled and assessed: legitimacy and instrumentality. The boundaries created by how decision makers perceive these two dimensions mean that only four selection patterns can be chosen by decision makers: hybridization, stratification, contamination or routinization.