The phrase stems from the 5th Amendment of the US Constitution; prohibiting individuals from being subject for the same offence to be "twice put in jeopardy of life and limb", and refers, in the strict sense, to three protections: protection from being retried for the same crime after an acquittal; protection from retrial after a conviction; and protection from being punished multiple times for the same offense. When employed strictly, the mechanism can be used as a defense - for example, the policemen who beat up black motorist Rodney King in 1991 in Los Angeles, CA, were acquitted of assault in a county court, and as a result, could not be tried for those crimes in Federal court, and mirror court cases in the Southern United states in the 1960s where racially motivated crimes were not actively prosecuted nor convicted in local courts. A more pronounced example is that of U.S. citizen and terrorist Timothy McVeigh; sentenced to death for murdering eight U.S. federal employees with a bomb (as federal law only covers the federal employees killed in the explosion, a state court could have tried him for the deaths of the other hundred. In 2003, Home Secretary David Blunkett abolished this strict form of double jeopardy; retrials are now allowed if there is 'new and compelling evidence'. In addition, an optional protocol (specifically, the Seventh Protocol, Article Four) of the European Convention of Human Rights, which protects against double jeopardy, states: "No one shall be liable to be tried or punished again in criminal proceedings under the jurisdiction of the same State for an offence for which he has already been finally acquitted or convicted in accordance with the law and penal procedure of that State."; only Belgium, Germany, The Netherlands, Portugal, Spain, and the United Kingdom have not ratified this optional convention.
contributed by user Gregory Block