Discontinuous-constituent Phrase Structure Grammar (DCPSG) (distinct from Discontinuous Phrase Structure Grammar/DPSG) is a formalism for describing discontinuous phrase structures in natural language, such as verb phrases in VSO languages. The formalism was introduced in the slightly more constrained form of Discontinuous-constituent Phrase Structure Grammar with Subscripts and Deletes (DCPSGsd) in Harman (1963).[1] DCPSGs describe a superset of the context-free languages, by means of rewrite rules that permit a limited amount of wrapping, similar to that found in Head grammar.
Rewrite rules of a DCPSG are identical to those of a CFG, with the addition of a meta-symbol, denoted here as an underscore. DCPSG rules therefore have the general form
X\to\alpha
\alpha
The rewrite semantics of DCPSG are identical as those of a CFG when the rule being used does not contain an underscore: given a rule
X\to\alpha
X
\alpha
For rules with an underscore, the rewrite semantics are slightly different: given a rule
X\to\alpha\\beta
X
\alpha
\beta
\beta
X
We can characterize the gross sentence structure of a VSO language such as Irish with the following rules (substituting English words for Irish words, and using subscripts solely for demonstration of discontinuity):
A derivation for the sentence saw John Susan, where John is the subject, and Susan is the direct object forming a VP with saw is: