This situation can occur only in common law legal systems, because civil law systems either do not use juries at all or provide that the defendant is immediately acquitted if the majority or supermajority required for conviction is not reached during a single, solemn vote.
In Australia, until the advent of majority juries, a unanimous verdict had to be reached in criminal trials. 
In Canada, the jury must reach a unanimous decision on criminal cases. If the jury cannot reach a unanimous decision, a hung jury is declared. A new panel of jurors will be selected for the retrial. Each jury in criminal courts contain twelve jurors. However this is not the case in civil cases. In civil cases, only six people are necessary for a jury, and if there is one dissenter but the rest are unanimous (i.e. a 5-1 vote) the one dissenter can be ignored with the majority opinion becoming the final verdict.
In New Zealand, the jury must initially try to reach a unanimous verdict. If the jury cannot reach a unanimous verdict after a reasonable time given the nature and complexity of the case (but not less than four hours), then the court may accept a majority verdict. In criminal cases, an all-but-one vote is needed (i.e. 11-1 with a full jury); in civil cases, a three-fourths vote is needed (i.e. 9-3 with a full jury).
If the jury fails to reach either a unanimous or majority verdict after a reasonable time, the presiding judge may declare a hung jury, and a new panel of jurors will be selected for a retrial. If the retrial also results in a hung jury, the case must be referred to the Solicitor-General. The Solicitor-General will generally issue a stay of proceedings unless there are compelling reasons to proceed with a third trial.
In England and Wales a majority of 10-2 (10-1 if only eleven jurors remain) is needed for a verdict; failure to reach this may lead to a retrial.
Initially, the jury will be directed to try to reach a unanimous verdict. If they fail to reach a unanimous verdict, the judge may later (after not less than two hours) give directions that a majority verdict will be acceptable, but still no less than ten to two, although the jury should continue to try to reach a unanimous verdict if possible.
When the jury is called to deliver a verdict after majority directions have been given, a careful protocol of questions is followed: only in the event of a guilty verdict is it then asked whether or not all jurors were agreed on that verdict, to prevent any acquittal from being tainted by it being disclosed that any jurors dissented. The protocol is followed separately for each charge.
It is not possible to have a hung jury in Scotland in criminal cases. Juries consist of 15, and verdicts are decided by simple majority (8) of the initial membership. If jurors drop out because of illness or another reason, the trial can continue with a minimum of 12 jurors, but the support of 8 jurors is still needed for a guilty verdict; anything less is treated as an acquittal.
In civil cases there is a jury of 12, with a minimum of 10 needed to continue the trial. It is possible to have a hung jury if there is a tied vote after three hours' deliberation.
In the United States, the result is a mistrial, and the case may be retried (United States v. Perez, 1824). Some jurisdictions permit the court to give the jury a so-called Allen charge, inviting the dissenting jurors to re-examine their opinions, as a last-ditch effort to prevent the jury from hanging. The Federal Rules of Criminal Procedure state, "The verdict must be unanimous. . . . If there are multiple defendants, the jury may return a verdict at any time during its deliberations as to any defendant about whom it has agreed. . . . If the jury cannot agree on all counts as to any defendant, the jury may return a verdict on those counts on which it has agreed. . . . If the jury cannot agree on a verdict on one or more counts, the court may declare a mistrial on those counts. A hung jury does not imply either the defendant's guilt or innocence. The government may retry any defendant on any count on which the jury could not agree."
Juries in criminal cases are generally, as a rule, required to reach a unanimous verdict, and juries in civil cases typically have to reach a majority on some level. If a defendant has been found guilty of a capital offense, one that could result in the death penalty if the person is eligible, the opinion of the jury must be unanimous if the defendant is to be sentenced to death. Currently, only one state, Oregon, does not require unanimous verdicts in all criminal cases. Oregon requires a 10-2 majority for most felony convictions, but unanimity for first degree murder.
In jurisdictions giving those involved in the case a choice of jury size (such as between a six-person and twelve-person jury), defense counsel in both civil and criminal cases frequently opt for the larger number of jurors. A common axiom in criminal cases is that "it takes only one to hang," referring to the fact that in some cases, a single juror can defeat the required unanimity.
One proposal for dealing with the difficulties associated with hung juries has been to introduce supermajority verdicts to allow juries to convict defendants without unanimous agreements amongst the jurors. Hence, a 12-member jury that would otherwise be deadlocked at 11 for conviction and 1 against, would be recorded as a guilty verdict. The rationale for majority verdicts usually includes arguments involving so-called 'rogue jurors' who unreasonably impede the course of justice. Opponents of majority verdicts argue that it undermines public confidence in criminal justice systems and results in a higher number of individuals convicted of crimes they did not commit.
In United States military justice, the Uniform Code of Military Justice (10 U.S.C. Chapter 47) Article 52 specifies the minimum number of court martial panel members required to return a verdict of guilty. In cases that involve a mandatory death sentence, a unanimous vote of all panel members is required. In cases that involve mandatory life sentences or sentences of confinement over ten years, a three-fourths vote is required. In all other cases, only a two-thirds vote is required to convict. Additionally, the Manual for Courts-Martial requires only a judge and a specified number of panel members (five for a general court-martial or three for a special court-martial; no panel is seated for a summary court-martial) in all non-capital cases. In capital cases, a panel of 12 members is required.
The only state which does not require a unanimous jury decision is Alabama. In Alabama, at least 10 jurors must concur. A retrial happens if the jury deadlocks.
Nebraska is the only state in which the sentence is decided by a three-judge panel. If one of the judges on the panel opposes death, the defendant is sentenced to life imprisonment.
Montana is the only state where the trial judge decides the sentence alone.
In all states in which the jury is involved, only death-qualified veniremen can be selected in such a jury, to exclude both people who will always vote for the death sentence and those who are categorically opposed to it.
The first outcome is referred as the "true unanimity" rule, while the third has been criticized as the "single-juror veto" rule.