This labelling phenomenon has limitations however. Some labels can combine and others cannot. For example, two lexical structure labels cannot directly combine. The two nouns, "Lucy" and "dress" cannot directly be combined. Likewise, neither can the noun "pencil" be merged with the adjective "short", nor can the verbs, "want" and "drink" cannot be merged without anything in between. As represented by the schematic below, all of these examples are impossible lexical structures. This shows that there is a limitation where lexical categories can only be one layer deep. However, these limitations can be overcome with the insertion of an expression layer in between. For example, to combine "John" and "book", adding a determiner such as "-'s" makes this a possible combination.