The opinion of the court was delivered by: Justice Thomas
 Docket No. 95725-Agenda 2-January 2004.
 The issue presented is whether an Apprendi claim that is waived pursuant to a guilty plea may nevertheless be considered on appeal under the plain error exception to the waiver rule. We hold that it may not.
 In exchange for the State seeking neither the death penalty nor a mandatory life sentence, defendant, Dennis Townsell, pleaded guilty to first degree murder (Ill. Rev. Stat. 1991, ch. 38, par. 9-1(a)(1)). A sentencing hearing was held, and the circuit court of Rock Island County imposed an extended-term sentence of 100 years in prison. The extended-term sentence was based on the trial court's finding that defendant's offense was "accompanied by exceptionally brutal or heinous behavior indicative of wanton cruelty" (Ill. Rev. Stat. 1991, ch. 38, par. 1005-5-3.2(b)(2)).
 On appeal, the appellate court reduced defendant's sentence to 60 years, finding that the imposition of an extended-term violated the United States Supreme Court's decision in Apprendi v. New Jersey, 530 U.S. 466, 147 L. Ed. 2d 435, 120 S. Ct. 2348 (2000). 336 Ill. App. 3d 340. In so ruling, the appellate court initially acknowledged that, under this court's decision in People v. Jackson, 199 Ill. 2d 286, 302 (2002), defendant's guilty plea waived any Apprendi-based claims. 336 Ill. App. 3d at 343. The court then held, however, that an Apprendi violation is so significant as to constitute plain error, which is an exception to the waiver rule under Supreme Court Rule 615(a) (134 Ill. 2d R. 615(a)). Consequently, the appellate court reached the merits of defendant's Apprendi claim, vacated his extended-term sentence, and imposed a nonextended term of 60 years in prison. 336 Ill. App. 3d at 346.
 We allowed the State's petition for leave to appeal. 177 Ill. 2d R. 315(a).
 It is well established that a voluntary guilty plea waives all non-jurisdictional errors or irregularities, including constitutional ones. People v. Peeples, 155 Ill. 2d 422, 491 (1993). In People v. Jackson, 199 Ill. 2d 286 (2002), this court was asked to decide whether the "errors or irregularities" waived by a guilty plea include Apprendi-based sentencing claims. In answering this question, Jackson began by acknowledging that, under Apprendi, "[e]very fact necessary to establish the range within which a defendant may be sentenced is an element of the crime and thus falls within the constitutional rights of a jury trial and proof beyond a reasonable doubt." Jackson, 199 Ill. 2d at 296. Nevertheless, by pleading guilty, "a defendant waives exactly those rights." (Emphasis in original.) Jackson, 199 Ill. 2d at 296. Indeed, "[a] knowing relinquishment of the right to a trial by jury is the sine qua non of a guilty plea." Jackson, 199 Ill. 2d at 296. Jackson therefore concluded that "Apprendi-based sentencing claims cannot be heard on appeal from a guilty plea." Jackson, 199 Ill. 2d at 296.
 In rejecting Jackson's seemingly unequivocal holding, the appellate court expressed its discomfort with the fact that, under Jackson, a defendant can be deemed to have waived an Apprendi-based claim even though he entered his guilty plea long before Apprendi was decided. According to the appellate court:
 "Townsell would prevail on his Apprendi claim if he had proceeded to trial rather than pleading guilty. He would prevail because extension of his sentence offended substantial constitutional rights. We cannot in good conscience reach a different result merely because he pled guilty. We realize that he relinquished the rights upon which Apprendi is based when he entered his plea. However, the dispositive nature of that relinquishment is questionable because the plea occurred long before Apprendi was decided. How can we say that Townsell really waived his right to a procedure that remained unprescribed for another decade?" 336 Ill. App. 3d at 345-46.
 For these reasons, the appellate court "opt[ed] to forgo the administrative convenience of a waiver" and addressed defendant's Apprendi argument on the merits. 336 Ill. App. 3d at 346.
 The fatal flaw in the appellate court's analysis rests in the assumption that the procedures outlined in Apprendi "remained unprescribed" until Apprendi was decided in 2000. Nothing could be further from the truth. In Jackson, this court clearly explained that:
 "Apprendi did not deal with novel constitutional rights. Rather, the Court was concerned with the applicability and reach of the well-established constitutional rights to a jury trial and proof beyond a reasonable doubt, rights which a guilty plea is specifically designed to waive." (Emphasis added.) Jackson, 199 Ill. 2d at 302.
 Similarly, in Hill v. Cowan, 202 Ill. 2d 151 (2002), this court specifically rejected "the complaint that a defendant did not `know' that he was waiving the right to have the State prove enhancing factors beyond a reasonable doubt, because by pleading guilty the defendant releases the State from proving anything beyond a reasonable doubt." (Emphasis in original.) Hill, 202 Ill. 2d at 154. Again, "[a] knowing relinquishment of the right to a trial by jury is the sine qua non of a guilty plea." Jackson, 199 Ill. 2d ...