Massachusetts Criminal Procedure Rule 19: Trial by Jury or by the Court
(Applicable to Superior Court and jury sessions in District Court)
(a) General. A case in which the defendant has the right to be tried by a jury shall so be tried unless the defendant waives a jury trial in writing with the approval of the court and files the waiver with the clerk, in which instance he shall be tried by the court instead of by a jury. If there is more than one defendant, all must waive the right to trial by jury, and if they do not so waive, there must be a jury trial unless the court in its discretion severs the cases. The court may refuse to approve such a waiver for any good and sufficient reason provided that such refusal is given in open court and on the record.
(b) Less Than a Full Jury. If after jeopardy attaches there is at any time during the progress of a trial less than a full jury remaining, a defendant may waive his right to be tried by a full jury and request trial by the remaining jurors by signing a written waiver which shall be filed with the court. If there is more than one defendant, all must sign and file a waiver unless the court in its discretion severs the cases.