Work on the Sixth Edition began in the fall of 1969, before the Fifth Edition was finalized. In contrast to the previous versions, where the specification documents were based on whatever changes had been made to the compiler, for the new version a complete specification was written beforehand. This version was worked on by Kemény and Kurtz, as well as several former students who returned as faculty; Stephen Garland, John McGeachie, and Robert Hargraves. It was allowed considerable time to mature, with a beta version running for three months during the summer of 1971, before it was finally released on 21 September 1971. As a result of giving the design time to mature, it was, as Kurtz described, "probably the best-designed and most stable software system Dartmouth has ever written."