On Monday, Woman's Hour ran a feature on the rule of thumb - the supposed law which states that, in England, it used to be legal, for a husband to beat his wife provided he used a stick no thicker than his thumb.
Despite numerous references to the law in books on domestic violence and social history, there is no Act of Parliament or official law report which refers to it.
Sara Elin Roberts, an expert on Welsh Law and History, attempts to unravel the mystery further. Listen to Monday's feature.
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