This is how language apparently works. For the moment, use S to represent a clause, although we will change this soon. When X contains two categories Y and Z, X is said to branch. Y and Z are sister nodes. That is, sister nodes must be contiguous.
Nodes belong to categories, identified by labels.
B. Phrases are built up from heads by adding phrases of the same category which contain that head and additional constituents. These higher phrases are called projections of the head. A specifier is a daughter of the maximal projection. Subject-Predicate agreement is a clausal property. We will need c-command in the future.