Abstract. We introduce the concept of communicability angle between a pair of nodes in a graph. We provide strong analytical and empirical evidence that the average communicability angle for a given network accounts for its spatial efficiency on the basis of the communications among the nodes in a network. We determine characteristics of the spatial efficiency of more than a hundred real-world complex networks that represent complex systems arising in a diverse set of scenarios. In particular, we find that the communicability angle correlates very well with the experimentally measured value of the relative packing efficiency of proteins that are represented as residue networks. We finally show how we can modulate the spatial efficiency of a network by tuning the weights of the edges of the networks. This allows us to predict effects of external stresses on the spatial efficiency of a network as well as to design strategies to improve important parameters in real-world complex systems.distance; graph planarity; Euclidean distance 1. Introduction. Graphs are frequently used to represent discrete objects both in abstract mathematics and computer sciences as well as in applications, such as theoretical physics, biology, ecology and social sciences [26,14]. In the particular case of representing the networked skeleton of complex systems, graphs receive the denomination of complex networks; we will hereafter use graphs and networks interchangeably.Complex networks are ubiquitous in many real-world scenarios, ranging from the biomolecular -those representing gene transcription, protein interactions, and metabolic reactions -to the social and infrastructural organization of modern society [11,36,9]. In many of these networks, nodes and edges are used to represent physically embedded objects [4], namely spatial networks. In urban street networks, for instance, the nodes describe the intersection of streets, which are represented by the edges of the graph. These streets and their intersections are embedded in the twodimensional space representing the surface occupied by the corresponding city [28]. Thus, these networks are planar graphs in the sense that we can draw them in a plane without edge intersections, except for the few bridges and overpasses present in a city. Another spatial network is the brain network, in which the nodes account for brain regions embedded in the three-dimensional space occupied by the brain, while the edges represent the communication or physical connections among these regions [7]. We can also capture the three-dimensional structure of proteins by means of the residue networks in which nodes describe amino acids and the edges represent physical interactions among them. Other examples include the following: infrastructures, such as the Internet, transportation networks, water and electricity supply networks, etc. [4]; anatomical networks, such as vascular and organ/tissue networks; the networks of channels in fractured rocks; the networks representing the corridors and galleries in animal nest...