The papers in this volume all explore one kind of functional explanation for various aspects of linguistic form - iconicity: linguistic forms are frequently the way they are because they resemble the conceptual structures they are used to convey, or, linguistic structures resemble each other because the different conceptual domains they represent are thought of in the same way. The papers in Part I of this volume deal with aspects of motivation, the ways in which the linguistic form is a diagram of conceptual structure, and homologous with it in interesting ways. Most of the papers in Part II focus on isomorphism, the tendency to associate a single invariant meaning with each single invariant form. The papers in Part III deal with the apparent arbitrariness that arises from competing motivations.