Lexical Functional Grammar (LFG) was developed by Kaplan and Bresnan in 1980’s. As the name suggests lexicon plays a major role in LFG. It claims having computational viability and cognition. LFG competes with Government and Binding theory.
LFG has two levels of processing. In the first level, phrase structure rules (CFGs) are used to get a tree structure. This tree structure, known as constituent structure or C structure, captures the linear and hierarchical relationships. Thereafter functional structure of F-structure is obtained by unifying feature bundles.
There are similarities between LFG and ATN. The CFG rules that generate C-structure are similar to the unaugmented part of ATN. The functional description of LFG are similar to the augmentation in ATN. So LFG grammars are as powerful as ATN grammars.
LFG employs double shafted arrow mechanism for dealing with long distance dependencies. The double shafted mechanism is less natural and less intuitive. LFG chose direct linking of functionally dependent constituents. . Also it uses single operation of unification thereby making the grammar easy to write. Unification, on the other hand, is a costly operation. Unification results in large number of feature structures built and then rejected.