Jury nullification is an example of common law, according to StreetInsider.com. Jury veto power occurs when a jury has the right to acquit an accused person regardless of guilt under the law.
Wikipedia claims that jury nullification took root in the common law courts of England during the Middle Ages. Jury veto power was more firmly established in the Magna Carta of 1215, which declared that a jury comprising of a defendant's peers must issue a verdict. Even though early jurors faced imprisonment or fines for delivering a wrong verdict, there are cases in which juries acquitted a person regardless of the law or court instruction.
StreetInsider.com notes the most notable jury veto case occurred over 300 years ago, when jurors refused to convict William Penn for preaching Quakerism. The jurors disregarded the law and ignored instructions from the judge to deliver a guilty verdict. The jurors were punished, but they addressed their grievances to another court. Their punishments were overturned, and Chief Justice John Vaughan ruled that jurors had every right to vote according to their convictions, which also set the stage for the freedoms of assembly, speech and religion. The founding fathers of the United States adopted these principles, including the English common law notion of jury nullification. Under the American legal system, jurors also cannot be punished for any verdicts handed down to a defendant, and they are not obligated to provide an explanation for their verdict.