Mass. Gen. Laws ch. 234A § 68
In every twelve-person jury case, the court shall impanel at least two additional jurors. In every six-person jury case, the court shall impanel at least one additional juror. Alternate jurors shall not be identified until immediately prior to jury deliberations in accordance with the following. If, at the time of the submission of the case by the court to the jury for its deliberations upon a verdict, more than the number of jurors required for deliberation are available, the court shall direct the clerk to place the names of all of the available jurors except the foreperson into a box or drum and to select at random the names of the appropriate number of jurors necessary to reduce the jury to the proper number of members required for deliberation in the particular case. The jurors so selected shall not be discharged, but shall be known as alternate jurors. The alternate jurors shall be kept separate from the jury in some convenient location, subject to the same rules and procedures as govern the jury during its deliberations, until the jury has agreed upon a verdict or has been otherwise discharged. If at any time after the submission of the case by the court to the jury for its deliberation upon a verdict, a juror is discharged by the court for any reason, the court shall direct the clerk to place all of the names of the alternate jurors in a box or drum and to select at random the name of an alternate juror. The alternate juror so selected shall take the place of the discharged juror on the jury. The jury, so constituted, shall begin its deliberations anew with full authority to render a verdict in the case. The court shall have jurisdiction to receive the verdict of the jury, as constituted under the provisions of this section, and shall have jurisdiction and full authority to render judgment in the case. Whenever it is appropriate for the court to direct a verdict, the court may do so without first reducing the number of jurors to the proper number required for deliberation in the case. Upon a finding of cause, the trial judge may impanel a lesser number of jurors than specified under this section. Nothing in this section shall prevent the court from rendering a valid judgment based upon a verdict rendered by fewer jurors than required under this section where all parties have by stipulation agreed to this procedure. Nothing in this section shall prevent the court from entering a valid judgment based upon a verdict rendered by fewer or more jurors than required under this section or based upon procedures other than that specified in this section where all parties have by stipulation agreed to such a number of jurors or to such procedures.
Mass. Gen. Laws ch. 234A, § 68