So far, we have mostly described an MDA process in terms of a single hop from one or more source models to a single target model. Of course, systems these days are often too complex to be transformed in a single hop, and we need to break down the mapping into several hops. The output of each hop, including the target models and the associated marks, can serve as inputs to other mappings. When we link two sets of mappings together, we have a mapping chain.
A mapping chain comprises a series of what we might think of as stepping stones. Each stone represents a model you've selected; each step from one stone to another represents a mapping function. The path from one side to the other, then, constitutes a particular mapping chain for your project. Deciding where to place the stones, and planning the journey from one side to the other, constitutes a definition for an MDA process.