Phrase markers, example?

[S [NP The cat] [VP [V sat] [PP [P on] [NP [Det the] [N mat]]]]]

This phrase marker represents the sentence "The cat sat on the mat."

Phrase markers are syntactic structures that represent the hierarchical relationships between words in a sentence. They are typically represented as trees, with the root node representing the main verb of the sentence and the other nodes representing its arguments.

For example, the following phrase marker represents the sentence "The boy kicked the ball."

```
S
/ \
NP VP
/ \ \
Det N V NP
| | | |
the boy kicked the ball
```

In this phrase marker, the root node is VP, which represents the verb phrase "kicked the ball." The NP node to the left of VP represents the subject of the sentence, "the boy." The NP node to the right of VP represents the object of the sentence, "the ball." The Det node represents the determiner "the," and the N node represents the noun "boy" and "ball."

Phrase markers are useful for representing the syntactic structure of sentences because they make it clear which words are related to each other and how. They can also be used to generate sentences by starting at the root node and working down the tree, replacing each node with a word that satisfies its syntactic constraints.