These grammar formalisms encode lexical descriptions as feature structures, with inheritance and unification as the two basic operations relating these structures to one another. Likewise, feature structures have a clear mathematical and computational interpretation and provide an ideal data structure to encode complex word knowledge information. Informally, a feature structure is a set of attribute-value pairs, where values can be atomic or feature structures themselves, providing a partial specification of words, affixes and phrases. Unification provides the means for integrating inherent and inherited specifications of feature structure descriptions. Consequently, a valid lexical entry can never contain conflicting values.
Unification thus provides a way to perform a consistency check on lexical descriptions.
For example, the danger of inadvertently assigning distinct orthographies or parts of speech to the same word entry is easily avoided as the unification of incompatible information leads to failure. Through typing, feature structures can be arranged into a closed hierarchy so that two feature structures unify only if their types have a common subtype. Typing is also used to specify exactly which attributes are appropriate for a given feature structure so that arbitrary extensions of feature structures are easily eschewed. A relaxation of monotonicity, however, is sometimes useful in order to capture regularities across the lexicon. It would therefore be convenient to state that all verbs inherit the same regular morphological paradigm by default and then let the idiosyncratic specifications of irregular verbs override inherited information which is incompatible.