When representing empty categories in tree structures, linguists use a null symbol (∅) to depict the idea that there is a mental category at the level being represented, even if the word(s) are being left out of overt speech.
In order to illustrate empty categories in tree structures, linguists utilize a placeholder symbol (∅), which represents the notion that there exists a mental category at the current level of representation, regardless of whether the words are being left out of spoken language.