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Generally the prosecution bears the burden of proof and is required to prove their version of events to this standard. This means that the proposition being presented by the prosecution must be proven to the extent that there could be no "reasonable doubt" in the mind of a "reasonable person" that the defendant is guilty. There can still be a doubt, but only to the extent that it would not affect a reasonable person's belief regarding whether or not the defendant is guilty. "The shadow of a doubt" is sometimes used interchangeably with reasonable doubt, but this extends beyond the latter, to the extent that it may be considered an impossible standard. Reasonable doubt is therefore used.
If doubt does affect a "reasonable person's" belief that the defendant is guilty, the jury is not satisfied beyond "reasonable doubt". The precise meaning of words such as "reasonable" and "doubt" are usually defined within jurisprudence of the applicable country.
The use of "reasonable doubt" as a standard requirement in the Western justice system originated in medieval England. In English common law prior to the "reasonable doubt" standard, passing judgment in criminal trials had severe religious repercussions for jurors. According to Christian law prior to the 1780s: "the Juryman who finds any other person guilty, is liable to the Vengeance of God upon his Family and Trade, Body and Soul, in this world and that to come." It was also believed "In every case of doubt, where one’s salvation is in peril, one must always take the safer way.... A judge who is in doubt must refuse to judge." It was in reaction to these "religious fears" that "reasonable doubt" was introduced in the late 17th century to English common law, thereby allowing jurors to more easily convict. Therefore the original use of the "reasonable doubt" standard was opposite to its modern use of limiting a juror's ability to convict.
However, juries in criminal courts in England are no longer customarily directed to consider whether there is "reasonable doubt" about a defendant's guilt. Indeed, a recent conviction was appealed after the judge had said to the jury "You must be satisfied of guilt beyond all reasonable doubt." The conviction was upheld but the Appeal Court made clear their unhappiness with the judge's remark, indicating that the judge should instead have said to the jury simply that before they can return a verdict of guilty, they "must be sure that the defendant is guilty".
The principle of 'beyond reasonable doubt' was expounded in: Woolmington v DPP  UKHL 1 
In Canada, the courts clearly believe that the expression "beyond a reasonable doubt" requires clarification for the benefit of the jury. The key ruling discussing this is R. v. Lifchus where one of the four issues addressed was whether a trial judge should provide a jury with an explanation of the expression. Following R. v. Brydon the Supreme Court of Canada, in R. v. Lifchus, answered in the affirmative. In part, they wrote "The correct explanation of the requisite burden of proof is essential to ensure a fair criminal trial..."
The Court then addressed the second of the four issues before it by providing a lengthy discourse on the elements of a proper charge to the jury regarding the concept of "reasonable doubt". The Court did not prescribe any specific wording to be used by a given trial judge, but rather made recommendations outlining elements that should be included in the charge as well as elements that should be avoided.
The elements that should be included in the charge were: 
The elements to be avoided in the charge were: 
In New Zealand, jurors are typically told throughout a trial that the offence must be proved "beyond reasonable doubt", and judges usually include this in the summing-up. There is no absolute prescription as to how judges should explain reasonable doubt to juries. Judges usually tell jurors that they will be satisfied beyond reasonable doubt if they "feel sure" or "are sure" that the defendant is guilty. In line with appellate court direction, judges do little to elaborate on this or to explain what it means.
Research published in 1999 found that many jurors were uncertain what "beyond reasonable doubt" meant. "They generally thought in terms of percentages, and debated and disagreed with each other about the percentage certainty required for 'beyond reasonable doubt', variously interpreting it as 100 per cent, 95 per cent, 75 per cent and even 50 per cent. Occasionally this produced profound misunderstandings about the standard of proof."
In the United States, juries must be instructed to apply the reasonable doubt standard when determining the guilt or innocence of a criminal defendant, but there is much disagreement as to whether the jury should be given a definition of "reasonable doubt." In Victor v. Nebraska, the U.S. Supreme Court expressed disapproval of the unclear reasonable doubt instructions at issue, but stopped short of setting forth an exemplary jury instruction.