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United States

In criminal law in federal courts and a minority of state court systems of the United States, a grand jury is convened to hear only testimony and evidence to determine whether there is a sufficient basis for deciding to indict the defendant and proceed toward trial. In each court district where a grand jury is required, a group of 16–23 citizens holds an inquiry on criminal complaints brought by the prosecutor to decide whether a trial is warranted (based on the standard that probable cause exists that a crime was committed), in which case an indictment is issued. In jurisdictions where the size of a jury varies, in general the size of juries tends to be larger if the crime alleged is more serious. If a grand jury rejects a proposed indictment the grand jury's action is known as a "no bill." If they accept a proposed indictment, the grand jury's action is known as a "true bill." Grand jury proceedings are ex parte: only the prosecutor and witnesses who the prosecutor calls may present evidence to the grand jury and defendants are not allowed to present mitigating evidence or even to know the testimony that was presented to the grand jury, and hearsay evidence is permitted. This is so because a grand jury cannot convict a defendant. It can only decide to indict the defendant and proceed forward toward trial. Grand juries vote to indict in the overwhelming majority of cases, and prosecutors are not prohibited from presenting the same case to a new grand jury if a "no bill" was returned by a previous grand jury. A typical grand jury considers a new criminal case every fifteen minutes. In some jurisdictions, in addition to indicting persons for crimes, a grand jury may also issue reports on matters that they investigate apart from the criminal indictments, particularly when the grand jury investigation involves a public scandal. Historically, grand juries were sometimes used in American law to serve a purpose similar to an investigatory commission.

Both Article III of the U.S. Constitution and the Sixth Amendment require that criminal cases be tried by a jury. Originally this applied only to federal courts. However, the Fourteenth Amendment extended this mandate to the states. Although the Constitution originally did not require a jury for civil cases, this led to an uproar which was followed by adoption of the Seventh Amendment, which requires a civil jury in cases where the value in dispute is greater than twenty dollars. However, the Seventh Amendment right to a civil jury trial does not apply in state courts, where the right to a jury is strictly a matter of state law. However, in practice, all states except Louisiana preserve the right to a jury trial in almost all civil cases where the sole remedy sought is money damages to the same extent as jury trials are permitted by the Seventh Amendment. Under the law of many states, jury trials are not allowed in small claims cases. The civil jury in the United States is a defining element of the process by which personal injury trials are handled.

In practice, even though the defendant in a criminal action is entitled to a trial by jury, most criminal actions in the U.S. are resolved by plea bargain. Only about 2% of civil cases go to trial, with only about half of those trials being conducted before juries.

In 1898 the Supreme Court held that the jury must be composed of at least twelve persons, although this was not necessarily extended to state civil jury trials. In 1970, however, the Supreme Court held that the twelve person requirement was a "historical accident", and upheld six-person juries if provided for under state law in both criminal and civil state court cases. There is controversy over smaller juries, with proponents arguing that they are more efficient and opponents arguing that they lead to fluctuating verdicts. In a later case, however, the court rejected the use of five-person juries in criminal cases. Juries go through a selection process called voir dire in which the lawyers question the jurors and then make "challenges for cause" and "peremptory challenges" to remove jurors. Traditionally the removal of jurors based on a peremptory challenge required no justification or explanation, but the tradition has been changed by the Supreme Court where the reason for the peremptory challenge was the race of the potential juror. Since the 1970s " scientific jury selection " has become popular.

Unanimous jury verdicts have been standard in US American law. This requirement was upheld by the Supreme Court in 1897, but the standard was relaxed in 1972 in two criminal cases. As of 1999 over thirty states had laws allowing less than unanimity in civil cases, but, until 2020, Oregon and Louisiana were the only states which have laws allowing less than unanimous jury verdicts for criminal cases (these laws were overturned in Ramos v. Louisiana). When the required number of jurors cannot agree on a verdict (a situation sometimes referred to as a hung jury), a mistrial is declared, and the case may be retried with a newly constituted jury. The practice generally was that the jury rules only on questions of fact and guilt; setting the penalty was reserved for the judge. This practice was confirmed by rulings of the U.S. Supreme Court such as in Ring v. Arizona, which found Arizona's practice of having the judge decide whether aggravating factors exist to make a defendant eligible for the death penalty, to be unconstitutional, and reserving the determination of whether the aggravating factors exist to be decided by the jury. However, in some states (such as Alabama and Florida), the ultimate decision on the punishment is made by the judge, and the jury gives only a non-binding recommendation. The judge can impose the death penalty even if the jury recommends life without parole.

There is no set format for jury deliberations, and the jury takes a period of time to settle into discussing the evidence and deciding on guilt and any other facts the judge instructs them to determine. Deliberation is done by the jury only, with none of the lawyers, the judge, or the defendant present. The first step will typically be to find out the initial feeling or reaction of the jurors to the case, which may be by a show of hands, or via secret ballot. The jury will then attempt to arrive at a consensus verdict. The discussion usually helps to identify jurors' views to see whether a consensus will emerge as well as areas that bear further discussion. Points often arise that were not specifically discussed during the trial. The result of these discussions is that in most cases the jury comes to a unanimous decision and a verdict is thus achieved. In some states and under circumstances, the decision need not be unanimous.

In a few states and in death penalty cases, depending upon the law, the trial jury, or sometimes a separate jury, may determine whether the death penalty is appropriate in "capital" murder cases. Usually, sentencing is handled by the judge at a separate hearing. The judge may but does not always follow the recommendations of the jury when deciding on a sentence.