ID/LP rules

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Generalized Phrase Structure Grammar (GPSG) uses ID-rules (immediate dominance rules) and LP-rules (linear precedence rules) instead of context-free phrase structure rules to describe hierarchical and sequential aspects of linguistic units separately.


Context-free phrase structure rules can be interpreted as well-formedness conditions for local trees. A rule like X \rightarrow Y_1 Y_2 ... Y_n licences exactly one tree. This tree is characterised by two facts: (i). The root labelled X (directly) dominates the nodes labelled Y_1 ... Y_n and (ii). The node labelled Y_i precedes node Y_{i+1} (for i = 1, ... n-1).

An ID-rule X \rightarrow Y_1, Y_2, ..., Y_n on the other hand, licences all trees in which X dominates the other categories, no matter in which order these categories appear. An LP-rule restrains the set of local trees licenced by an ID-rule: Each tree which violates the LP-rule is disregarded.


Replacing context-free rules by ID- and LP-rules was motivated by the idea that by this division of labor it is possible to state important generalization concerning the word order (constituent order) for a given natural language explicitly.

Each ID/LP-syntax can be converted into a strongly equivalent context-free phrase structure syntax. As the linear restrictions expressed by LP-rules are interpreted globally (no local tree licenced by any ID-rule may violate any of the LP-rules), there are context-free grammars which can not be converted into a strongly equivalent ID/LP-grammar (though into a weakly equivalent ID/LP-syntax).

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