Feature structures are the building blocks of unification grammars, as they serve as the counterpart of the terminal and nonterminal symbols in CFGs. However, in order to define grammars and derivations, one needs some extension of feature structures to sequences thereof. In this chapter we present multirooted feature structures that are aimed at capturing complex, ordered information and are used for representing rules and sentential forms of unification grammars; we motivate this extension in Section 4.1. In parallel to the exposition of feature structures in Chapter 2, we start by defining multirooted feature graphs (Section 4.2), a natural extension of feature graphs. We then abstract away from the identities of nodes in the graphs in two ways: by defining multirooted feature structures, which are equivalence classes of isomorphic multirooted feature graphs, and by defining abstract multirooted structures (Section 4.3). Finally, we define the concept of multi-AVMs (Section 4.4), which are an extension of AVMs, and show how they correspond to multirooted graphs. The crucial concept of unification in context is discussed in Section 4.5.

We then utilize this machinery for defining unification grammars. We begin by defining (sentential) forms and grammar rules (Section 4.6). Then, we define the concept of derivation for unification grammars, providing a means for defining the languages generated by such grammars (Section 4.7). We explore derivation trees in Section 4.8.

The move from context-free grammars to unification grammars is motivated by linguistic considerations (the need to provide better generalizations and more compact representations).