In the 1942 case of Adams, Warden, et al. v. United States ex rel. McCann, the U.S Supreme Court ruled that a defendant's waiver of his right to trial by jury was not valid if he did not fully understand the implications and consequences of such a decision. The court held that for such waivers to be valid they must be made competently and intelligently with full understanding on part of the accused person about their rights under law and potential repercussions from waiving them off. This ruling came in response to an appeal filed by Edward J McCann who had been convicted without a jury trial after waiving his right for one during proceedings against him related to mail fraud charges.

In the dissenting opinion for Adams, Warden, et al. v. United States ex rel. McCann (1942), Justice Frankfurter argued that a defendant's waiver of his right to trial by jury should not be easily presumed and must be clearly demonstrated in order to protect the fundamental rights guaranteed under the Constitution. He emphasized that such waivers should only be accepted when they are made knowingly and voluntarily, with full understanding of their implications on part of defendants themselves rather than through mere formalities or procedures followed by courts or legal representatives. Furthermore, he contended that it is crucial for courts to ensure this standard is met before proceeding with trials without juries as failure to do so would undermine public confidence in judicial processes and potentially lead to miscarriages of justice.