Rule of six
The rule of six is a law enacted by Queen Rhaenys Targaryen during the reign of Aegon I Targaryen. While holding court at the Aegonfort in King's Landing, a man was brought before the queen for beating his wife to death. The woman's brothers wanted the culprit punished, but the husband argued he was within his lawful rights, since he had caught his wife cheating on him and he hit her with a rod no bigger than his thumb. The right of a husband to chastise an erring wife was well established in the Seven Kingdoms and the doctrines of the Faith of the Seven. When Rhaenys asked him how many times he had hit her, the husband could not recall, but the woman's brothers insisted it was a hundred blows.
Rhaenys, wishing not to offend the Faith, consulted with several septons and maesters, then decreed that as there are seven gods, and an adulterous wife was an insult to the Seven, he was within his rights to punish her, but only with six blows, for the seventh blow represented the Stranger, who was death. She ruled that the man was to be punished, as the first six blows were lawful, but the other ninety-four were against the law. Rhaenys decreed that the murdered woman's brothers would deliver the man ninety-four blows with lawful rods no bigger than their thumbs. Another ruling, known as the rule of thumb, was put in place also, making it against the law to strike an adulterous wife with any rod or implement bigger than the husband's thumb. 
- Fire & Blood, Three Heads Had the Dragon - Governance Under King Aegon I.