There’s nothing magical about the number 12, except that 12-person juries enjoy a long and rich history in the law.
Although informal use of juries pre-dates the 12th century, King Henry II (1154-1189) is credited with instituting the first official jury system, when in 1166 he ordered that “12 lawful men” in each village would make decisions about who had committed crimes in their community.* Since that time, legal systems with roots in English common law (United States, Austrialia, Canada, etc., including 49 of the 50 states in the U.S.) use a jury system to some degree, although the precise number comprising the jury used will vary from jurisdiction to jurisdiction.
In 1970, the U.S. Supreme Court approved the use of a 6-person jury in a Florida criminal case, ruling that neither the language nor the history of the U.S Constitution mandates a 12-person jury. Instead, the Supreme Court, referring to the 12-person jury as a “historical accident,” held that the purpose of a jury is to provide a cross-section of the community, and juries of less than 12 persons in serious felony cases do not violate that purpose or the constitution. Nevertheless, only two states in the U.S. (Florida and Connecticut) allow for 6-person juries for serious felony accusations.
In Texas, both statutory and state constitutional law provides that juries will be composed of either 12 or 6 persons, depending upon the type of court which is empanelling the jury. Basically, district-level trial court cases are decided by 12-person juries, while cases from county-level trial courts, justice courts and municipal court are decided by juries of 6. (In district courts, however, the parties may agree to reduce the number of jurors to hear a particular case.) What this means for criminal cases is that a felony case will be decided by a 12-person jury, while a misdemeanor case will be determined by a jury of 6.
(*King Henry II’s jury system was a positive departure from the common practice of deciding guilt or innocence during that era. Prior to this time, criminal cases were often adjudicated through “trial by ordeal.” In a “trial by ordeal,” those accused of crimes were required to undertake some type of ordeal, such as picking up a red-hot bar of iron or sticking their hands into a boiling cauldron. If their injuries from the ordeal quickly healed, that would be considered a sign from God that they were innocent. If not, a criminal penalty would be assessed – a textbook example of adding insult to injury!)
— Bonnie Sudderth, Judge of the 352nd District Court of Tarrant County, Texas