By the 13th century in England, if an accused person could not produce 12 neighbors -- the original jury of his peers -- to swear to his innocence, he was presumed guilty. This system was especially problematic for a stranger passing through town or someone lacking in social skills.

Such an Ordeal

Still, it was usually preferable to two other 13th century English alternatives, the ordeal and the duel. In one version of the ordeal, the accused had to carry a red-hot iron for a short distance. Only if he was healed on the third day was he judged innocent. In the duel, some well-to-do defendants would hire champions to fight for them, a practice handed down to the modern era in the form of retaining a lawyer.

Other countries in medieval Europe turned to juries that ranged in size from four to 66. In modern Western Europe, several countries allow juries of a mere handful, with a mix of lay people and judges.

And in 1970, the United States Supreme Court ruled that the Constitution does not require that a jury be made up of 12 people; six would do just fine. For cases other than capital offenses, Arizona allows an eight-member jury and Florida allows six. In a 1978 Georgia case, however, the Supreme Court said a jury of five was cutting it too close.

With the prosecution's case not even concluded, the Simpson jury seems likely to dip below the magic number. In that event, an unsettled question in California law is what will happen if the defense agrees to proceed, but the prosecution does not.