Ritchie is best known as the creator of the C programming language, a key developer of the Unix operating system, and co-author of the book "The C Programming Language"; he was the 'R' in "K&R" (a common reference to the book's authors Kernighan and Ritchie). Ritchie worked together with Ken Thompson, who is credited with writing the original version of Unix; one of Ritchie's most important contributions to Unix was its porting to different machines and platforms. They were so influential on Research Unix that Doug McIlroy later wrote, "The names of Ritchie and Thompson may safely be assumed to be attached to almost everything not otherwise attributed."