In all of this research, he illuminated the fundamental importance of semantics, and its role in motivating syntactic and morphological phenomena. His collaboration with Paul Kay and George Lakoff was generalized into the theory of Construction Grammar. This work aimed at developing a complete theory of grammar that would fully acknowledge the role of semantics right from the start, while simultaneously adopting constraint-based formalisms as popular in computer science and natural language processing. This theory built on the notion of construction from traditional and pedagogical grammars rather than the rule-based formalisms that dominate most of generative grammar. One of Fillmore's most widely noticed works of the time (with Paul Kay and Cathy O'Connor) appeared in 'Language' in 1988 as "Regularity and Idiomaticity in Grammatical Constructions: The Case of Let Alone". Their paper highlighted the merits of such a theory of by focusing on the 'let alone' construction. Over time, construction grammar developed into a research area of its own, and a number of variants have been proposed over the years by different researchers.