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Perjury is the act of lying or making verifiable false statements under oath, especially when it happens in a law court. Perjury can be a crime because the witness has sworn to tell the truth, and for the credibility of the court, witness testimony must be relied on as being truthful. Not all mistatements of fact are perjury however, people make inaccurate statements all the time, they may do so in an inadvertent manner. An individual may have an honest but mistaken belief about a certain fact, or the individual's recollection may be inaccurate. Like all other crimes in the common law system perjury requires one to have the mens rea and actus reus necessary to prove the elements of a crime.
In some countries such as France, suspects cannot be heard under oath and thus do not commit perjury, whatever they say during their trial.
Perjury also applies to witnesses who affirm they are telling the truth, rather than swear an oath or in a statement that is made under penalty of perjury even if it has not been sworn or affirmed to before a notary public or other public officer authorized under law to take oaths.