Illformedness

Illformedness is a term used in linguistics to describe strings that fail to conform to the rules of a language’s grammar. It is inherently relative: a form can be ill-formed under one theory, dialect, or set of grammatical constraints and well-formed under another. Illformedness contrasts with well-formedness, which refers to strings that fit the permitted structures of a given grammar.

In practice, ill-formedness is most often discussed with respect to syntax, where a sentence may violate rule systems governing word order, agreement, subcategorization, and long-distance dependencies. Phonotactic constraints in phonology can also render sequences ill-formed if the sound combinations are not permitted in the language. Semantics may tolerate forms that are syntactically odd, but the focus of illformedness generally centers on grammatical structure rather than meaning.

The concept is closely tied to the distinction between competence and performance in linguistic theory. A string might be ill-formed according to a particular grammatical model, even if a speaker produces it in performance due to processing difficulty, speech errors, or deliberate experimentation. Conversely, dialectal variations or different sociolects can license forms that are ill-formed under another standard grammar.

Assessment of illformedness relies on native speaker judgments, acceptability experiments, and corpus evidence. It is used to diagnose the limits of a theory, identify constraints on grammar, and compare how different linguistic frameworks account for restrictions on permissible structures. In related fields such as logic and computer science, the term ill-formed is used similarly to denote strings that fail formal syntactic rules.