We’re about to start looking into how sentences are organized in our mental grammar. Before we do that, we need to be familiar with a particular kind of notation called a tree diagram. We’ll see that, within each sentence, words are grouped into phrases. Phrases can be grouped together to form other phrases, and to form sentences. We use tree diagrams to depict this organization. They’re called tree diagrams because they have lots of branches: each of these little lines that join things in the diagram is a branch. Within a tree diagram, we can talk about the relationships between different parts of the tree.
Every place where branches join together is called a node. Each node corresponds to a set of words that act together as a unit called a constituent, which we’ll talk about later in this chapter.
Each branch connects one node to another. The higher node is called the parent and the lower one is the child. A parent can have more than one child, but each child has only one parent. And, as you might expect, if two child nodes have the same parent, then we say that they’re siblings to each other. (Just so you know, most linguistics textbooks call these nodes “mother, daughter and sister” nodes, but we’re using non-gendered terms in this book.)
If a node has no children, we call it a terminal node.
Having this vocabulary for tree diagrams will allow us to talk about the syntactic relationships between the parts of sentences in our mental grammar.