The tree in (74) says that the whole phrase is an NP (NP) which has two branches. On the left branch is NP which immediately dominates the two conjoined NPs, and on the right branch is the N uncle, the head noun of the phrase. Numbering the phrases in this way is always optional, although it is often when there are several phrases of the same category, as in (74), and we need to make it clear which phrase we’re referring to