Why edges are needed

The term edge has become ubiquitous in syntactic theory in recent years: one reads of the left edge of the clause or the edge of VP in cartographic approaches, of the edge of phases in phase-based computations, of edge features driving movement, and even of edge features underlying merge. In this contribution I look at all these different meanings of edge and conclude that they all reduce to a property underlying labelling at the heart of structure building. That is to say, the reason edges are needed is the same as the reason why labels are needed. Since the necessity of labels has been disputed in the recent minimalist literature, I reassert the need for labels and attempt to derive it from interface legibility conditions.