In the mid-sixteenth century, three rabbis, one living in the Land of Israel, another in Poland, and a third in Lithuania, were independently trying to do what few had ever succeeded in doing: establish a single code of Jewish law. To accomplish this, they had to choose between a myriad of competing understandings of Jewish law (halakah) that had emerged on almost every topic during the previous millennium. How they did so is the subject of this work.