Introduction: How can the same English relationship 'in' apply to scenes as different as "the shoe in the box" (small, hollow, closed volume), "the bird in the tree" (large, dense, open volume) or "the fruit in the bowl" (curved surface)? What is the common 'across' invariant behind "he swam across the lake" (smooth trajectory, irregular surface) and "the fly zigzagged across the hall" (jagged trajectory, regular volume)? How can language, especially its spatial elements, be so insensitive to wide topological and morphological differences among visual percepts? In short, how does language drastically simplify information and categorize? The previous chapter addressed the basic concept of things and introduced algorithms dealing with image segmentation, perceptual mereology and object constituency. We now turn to the second major component of Langacker's trilogy, the relations between things. Relations clearly constitute the most important problem at the core of all theories of language. Ultimately, different theoretical perspectives on syntax will be distinguished on the basis of how they construe relations. It is only after relations are expressed in a mathematical form that processes can be modeled as temporal evolutions of relations and events as changes occurring during these processes.