On July 26, 1972, in People v. Tanner, the Michigan Supreme Court ruled that a minimum prison sentence could no longer exceed two-thirds of the maximum sentence. The ruling had several effects on Michigan's criminal justice system. It shifted substantial power over sentencing from trial judges to the parole board, reduced the average minimum sentence, altered plea bargaining strategies, and increased the power and prestige of the Michigan Supreme Court. The decision, designed to strengthen indeterminate sentencing, was delivered at the end of an era. As Tanner appeared, the first rumblings of discontent with indeterminate sentencing were being heard, and soon the main thrust of thought in this area would favor some form of definite sentencing. An impact study of Tanner is valuable, nevertheless, for its methodological contribution and its examination of criminal justice policymaking. The criminal procedures followed by the Michigan courts are briefly described, and the legal history and arguments leading to the Tanner decision are examined. The political environment that made the ruling possible and the resulting effects of Tanner on the supreme court's power are also considered. The results of regression analyses estimating the impact of the decision on sentencing are described, and the future impact of Tanner and its relevance in the ongoing indeterminate sentencing debate are assessed. It is generally concluded that in order for sentencing policies to be effective, social and empirical dynamics as well as legal considerations must be considered. Footnotes and tabular data are provided. (Author abstract modified)