It becomes more clear that neither of these two systems can exist alone with regards to human language when we look at the phenomenon of 'labeling'. This phenomenon refers to how we classify the grammatical category of phrases, where the grammatical category of the phrase is dependent on the grammatical category of one of the words within the phrase, called the head. For example, in the phrase "buy the books", the verb "buy" is the head, and we call the entire phrase a verb-phrase. There is also a smaller phrase within this verb-phrase, a determiner phrase, "the books" because of the determiner "the". What makes this phenomenon interesting is that it allows for hierarchical structure within phrases. This has implications on how we combine words to form phrases and eventually sentences.