In common law, a petit jury hears the evidence in a trial as presented by both the plaintiff (petitioner) and the defendant (respondent). After hearing the evidence and often jury instructions from the judge, the group retires for deliberation, to consider a verdict. The majority required for a verdict varies. In some cases it must be unanimous, while in other jurisdictions it may be a majority or supermajority. A jury that is unable to come to a verdict is referred to as a hung jury. The size of the jury varies; in criminal cases involving serious felonies there are usually 12 jurors, although Scotland uses 15. A number of countries that are not in the English common law tradition have quasi-juries on which lay judges or jurors and professional judges deliberate together regarding criminal cases. However, the common law trial jury is the most common type of jury system.