Feature cooccurrence restrictions
Generalized Phrase Structure Grammar (GPSG) uses Feature cooccurrence restrictions (FCRs) to formulate well-formedness conditions for feature structures. As GPSG takes feature structures to be arbitrary sets of feature specifications, it is necessary to block the combination of feature specifications which from a linguistic point of view make no sense. Most FCRs are formulated as implications.
Many theories of phonological features also apply FCRs.
Example
Only verbal catgories can contain the feature vform: [VFORM] <math>\rightarrow</math> [+V, -N]
Phrasal catgories cannot contain the feature subcat: [BAR 2] <math>\rightarrow</math> <math>\neg</math> [SUBCAT]
The vocalic fatures [+high] and [+low] cannot cooccur: [+high] <math>\rightarrow</math> [-low] && [+low] <math>\rightarrow</math> [-high]
Comments
Modern unification-based grammar formalisms like Head-Driven Phrase Structure Grammar use typed feature structures instead.
