An ID/LP grammar is a formal grammar that distinguishes immediate dominance (ID) constraints from linear precedence (LP) constraints. Whereas traditional phrase structure rules incorporate dominance and precedence into a single rule, ID/LP maintains separate rule sets which need not be processed simultaneously. ID/LP grammars are used in computational linguistics.
For example, a typical phrase structure rule might say S —> NP VP, indicating that an S-node dominates an NP-node and a VP-node, and that the NP precedes the VP in the surface string. In ID/LP grammars, this rule would only indicate dominance, and a separate rule indicating linear precedence, such as , would also be given.
The idea first came to prominence as part of generalized phrase structure grammar; the ID/LP approach is also used in head-driven phrase structure grammar, lexical functional grammar, and other unification grammars.
Current work in the Minimalist Program also attempts to distinguish between dominance and ordering. For instance, some versions specify that ordering is always Specifier-Head-Complement (a template overlaid on top of an unordered tree), or even derivable using more abstract structural relations within the tree (the Linear Correspondence Axiom).
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“Grammar is a tricky, inconsistent thing. Being the backbone of speech and writing, it should, we think, be eminently logical, make perfect sense, like the human skeleton. But, of course, the skeleton is arbitrary, too. Why twelve pairs of ribs rather than eleven or thirteen? Why thirty-two teeth? It has something to do with evolution and functionalism—but only sometimes, not always. So there are aspects of grammar that make good, logical sense, and others that do not.”
—John Simon (b. 1925)