A lexical rule is in a form of syntactic

Syntax

In linguistics, syntax is the study of the principles and rules for constructing phrases and sentences in natural languages....

 rule used within many theories of natural language syntax. These rules alter the argument structures of lexical

Lexicon

In linguistics, the lexicon of a language is its vocabulary, including its words and expressions. A lexicon is also a synonym of the word thesaurus. More formally, it is a language's inventory of lexemes. Coined in English 1603, the word "lexicon" derives from the Greek "λεξικόν" , neut...

 items (for example verb

Verb

A verb, from the Latin verbum meaning word, is a word that in syntax conveys an action , or a state of being . In the usual description of English, the basic form, with or without the particle to, is the infinitive...

s and declension

Declension

In linguistics, declension is the inflection of nouns, pronouns, adjectives, and articles to indicate number , case , and gender...

s) in order to alter their combinatory properties.

Lexical rules affect in particular specific word classes and morpheme

Morpheme

In linguistics, a morpheme is the smallest semantically meaningful unit in a language. The field of study dedicated to morphemes is called morphology. A morpheme is not identical to a word, and the principal difference between the two is that a morpheme may or may not stand alone, whereas a word,...

s. Moreover, they may have exceptions, do not apply across word boundaries and can only apply to underlying forms.

An example of a lexical rule in spoken English is the deletion of /n/. This rule applies in damn and autumn, but not in hymnal. Because the rule of n-deletion apparently needs information about the grammatical status of the word, it can only be lexical.

Lexical rules are the inverse of postlexical rules.