In the previous chapter, we defined the concept of a micro unit as an individual carrier of a generic rule and a micro trajectory as the process by which the rules of a micro unit change. In this chapter, we define a meso unit as a population of carriers of a rule and the trajectory or process by which the population emerges as successive adoption of a generic rule. A meso unit is therefore composed of a generic rule, a population of micro units and is the result of an ensemble of micro trajectories. This complex process is a meso trajectory. Yet as population, it forms a single unit in the analysis of coordination of the whole economy, which is a “macro” analysis of the coordination of all meso units.