  Many biological and man-made networked systems are characterized by the
simultaneous presence of different sub-networks organized in separate layers,
with links and nodes of qualitatively different types. While during the past
few years theoretical studies have examined a variety of structural features of
complex networks, the outstanding question is whether such features are
characterizing all single layers, or rather emerge as a result of
coarse-graining, i.e. when going from the multilayered to the aggregate network
representation. Here we address this issue with the help of real data. We
analyze the structural properties of an intrinsically multilayered real
network, the European Air Transportation Multiplex Network in which each
commercial airline defines a network layer. We examine how several structural
measures evolve as layers are progressively merged together. In particular, we
discuss how the topology of each layer affects the emergence of structural
properties in the aggregate network.
