5.2 Phrase structure rules and tree diagrams

Phrase Structure Rules and Tree Diagrams

Phrase structure rules describe how words combine into phrases and how phrases combine into sentences. They give you a formal system for representing the grammar of a language, and they're the foundation for drawing tree diagrams, which visually map out sentence structure.

Understanding these rules matters because they reveal that sentences aren't just strings of words in a row. Words group together into phrases, and those phrases nest inside each other in a hierarchy. Once you see that hierarchy, you can explain why certain word orders sound grammatical and others don't.

Phrase Structure Rules

What phrase structure rules do

A phrase structure rule is a rewrite rule that says "this category can be expanded into these parts." For example:

• S → NP VP means a sentence consists of a noun phrase followed by a verb phrase.
• NP → Det N means a noun phrase can be a determiner followed by a noun.
• VP → V NP means a verb phrase can be a verb followed by a noun phrase.

These rules capture the hierarchical structure of sentences. Rather than just listing words left to right, they show which words group together as a unit (a constituent) and how those units nest inside larger units.

They also let linguists generate grammatically correct sentences and compare syntactic patterns across different languages. Two languages might have different word orders but share similar phrase structure logic.

Applying phrase structure rules

You can use phrase structure rules in two directions: building sentences up, or breaking them down.

Building a sentence (top-down):

1. Start with the sentence symbol S.
2. Apply a rule to expand S (e.g., S → NP VP).
3. Expand each phrase using further rules (e.g., NP → Det N, VP → V NP).
4. Keep expanding until every symbol is a word (a terminal node).

For example, using S → NP VP, NP → Det N, and VP → V NP, you could generate: "The cat chased the mouse."

Breaking a sentence down (bottom-up): Start with the actual words, group them into phrases using the rules in reverse, and work your way up to S. If you can't get back to S following the rules, the sentence may be ill-formed.

This two-way process is also how you test whether a string of words is a valid sentence in the grammar. If the rules can't produce it, something is off structurally.

Tree Diagrams

Constructing a syntactic tree diagram

A tree diagram is the visual version of phrase structure rules applied to a specific sentence. Here's how to build one:

1. Write the root node S at the top.
2. Apply the first rule (e.g., S → NP VP) and draw branches down to NP and VP.
3. Expand each node using the appropriate rule. For NP → Det N, draw branches from NP down to Det and N.
4. Continue until you reach terminal nodes, which are the actual words of the sentence.
5. Arrange everything left-to-right so the word order matches the sentence as spoken.

Internal nodes are labeled with phrasal categories (NP, VP, PP). Terminal nodes are labeled with lexical categories (N, V, Det, Adj, P) and sit directly above the words themselves.

Reading relationships in tree diagrams

Once a tree is drawn, you can read several important structural relationships from it:

• Dominance: A node dominates another if you can trace a path downward from it. S dominates everything in the tree. Immediate dominance means a node is directly one level above another with no nodes in between.
• Sisterhood: Two nodes are sisters if they share the same parent node. In S → NP VP, the NP and VP are sisters.
• Constituency: A node and everything it dominates form a constituent. You can verify constituents using tests like substitution (can you replace the group with a single pronoun?), movement (can you move the group as a unit?), or coordination (can you join it with and to a similar group?).

Structural ambiguity

Sometimes a single sentence can be parsed with more than one tree diagram, which means the sentence is structurally ambiguous. A classic example: "I saw the man with the telescope." One tree attaches "with the telescope" to the VP (you used the telescope to see), and another attaches it to the NP (the man had the telescope). Drawing both trees makes the two interpretations explicit.