Transformational grammar is a theory of grammar that focuses on how sentences can be transformed into different structures while maintaining their meaning. Developed by Noam Chomsky in the 1950s, it emphasizes the underlying rules that govern sentence formation in a language. This approach helps linguists understand how people generate and comprehend sentences. The theory distinguishes between deep structure and surface structure. The deep structure represents the core meaning of a sentence, while the surface structure is the specific way that meaning is expressed. Transformational rules allow for the conversion between these two structures, illustrating the flexibility of language.