 Interestingly, many real-world networks such as social networks, the World Wide Web, and regulatory networks in biology display scale-free architectures. This raises important questions about whether real-world networks can be related to more generic structural types with characteristic functional capacities. For example, the scale-free structure makes the average path length between any two nodes in the network small, and this structure has therefore been associated with capacities for coordinated regulation throughout the network (Albert et al. 2000). Moreover, scale-free networks exhibit a high error tolerance, understood as robustness against failure of random nodes and links (e.g., random gene deletion). These functional properties are not shared by exponential networks, and illustrate some advantages of the scale-free structure. However, the functional importance of hubs in scale-free networks at the same time results in fragility to attacks on central notes (Albert et al. 2000; Barabási &amp; Oltvai 2004). Similarly, a multi-layered network structure called a bow-tie network connects many inputs and outputs through a central core (the knot of the bow-tie) that consists of a small number of elements (Kitano &amp; Oda 2006). This structure has been associated with efficient information flow, but also with fragility towards perturbations of the intermediate nodes in the core of the network.