In his firsthand history, “The Development of the C Language,” Dennis Ritchie writes that C “came into being in the years 1969-1973, in parallel with the early development of the Unix operating system,” and that “the most creative period occurred during 1972.” For that reason 1972 is commonly cited as the year C took shape as a usable language.
The milestone is best understood as a process rather than a single dated release. Ritchie’s account traces C’s evolution from the typeless language BCPL, through a language called B, to C with its type structure, all on the small DEC PDP-11 at Bell Labs. The work was driven by the practical need for a language in which to write the growing Unix system.
The significance of this period is large out of proportion to its quiet beginnings. A language built to implement one operating system on one minicomputer became the foundation for operating systems, compilers, and applications across the industry, and the direct ancestor of much later language design.