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Food-chain length is a central characteristic of ecological communities because of its strong influence on community structure and ecosystem function. While recent studies have started to better clarify the relationship between food-chain length and environmental gradients such as resource availability and ecosystem size, much less progress has been made in isolating the ultimate and proximate mechanisms that determine food-chain length. Progress has been slow, in part, because research has paid little attention to the proximate changes in food web structure that must link variation in food-chain length to the ultimate dynamic mechanism. Here we outline the structural mechanisms that determine variation in food-chain length. We explore the implications of these mechanisms for understanding how changes in food-web structure influence food-chain length using both an intraguild predation community model and data from natural ecosystems. The resulting framework provides the mechanisms for linking ultimate dynamic mechanisms to variation in food-chain length. It also suggests that simple linear food-chain models may make misleading predictions about patterns of variation in food-chain length because they are unable to incorporate important structural mechanisms that alter food-web dynamics and cause non-linear shifts in food-web structure. Intraguild predation models provide a more appropriate theoretical framework for understanding food-chain length in most natural ecosystems because they accommodate all of the proximate structural mechanisms identified here.