sentences:  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.The phenomenon was named and outlined by Noam Chomsky in his 1981 framework, and serves to address apparent violations of locality of selection — there are different types of empty categories that each appear to account for locality violations in different environments.Empty categories come in two major types