The majority continued: A district court imposes a sentence that is “based on” a Guidelines range for purposes of §3582(c)(2) if the range was a basis for the court’s exercise of discretion in imposing a sentence. Given the standard legal definition of “base,” there will be no question in the typical case that the defendant’s Guidelines range was a basis for his sentence. A district court is required to calculate and consider a defendant’s Guidelines range in every case. §3553(a).
Indeed, the Guidelines are “the starting point for every sentencing calculation in the federal system.” Peugh, supra, at 542. Thus, in general, §3582(c)(2) allows district courts to reconsider a prisoner’s sentence based on a new starting point—that is, a lower Guidelines range— and determine whether a reduction is appropriate. A sentence imposed pursuant to a Type-C agreement is no exception to the general rule that a defendant’s Guidelines range is the starting point and a basis for his ultimate sentence.
The Government and the defendant may agree to a specific sentence, but the Sentencing Guidelines prohibit district courts from accepting Type-C agreements without first evaluating the recommended sentence in light of the defendant’s Guidelines range. So in the usual case the court’s acceptance of a Type-C agreement and the sentence to be imposed pursuant to that agreement are “based on” the defendant’s Guidelines range. Since the Guidelines are a district court’s starting point, when the Commission lowers the range, the defendant will be eligible for relief under §3582(c)(2) absent clear demonstration, based on the record as a whole, that the court would have imposed the same sentence regardless of the Guidelines. This interpretation furthers §3582(c)(2)’s purpose, as well as the broader purposes of the Sentencing Reform Act. It is also reinforced by Molina-Martinez and Peugh, which both confirm that the Guidelines remain a basis for almost all federal sentences
Note that if the Commission were to change the Guidelines to make for a longer sentence, defendants would almost certainly be protected from a change that increases their prison term post-sentence. That would be a double jeopardy violation.