The legacy is an immensely complicated and quasi-legal series of arrangements, made largely through the banks holding the money, which establishes a fortune that cannot be manipulated by any one country's inheritance laws. Essentially it conserves the bulk of the Mayfair money and property in the hands of one person in each generation, this heir to the fortune being designated by the living beneficiary, except that should the beneficiary die without making the designation, the money goes to the eldest daughter. Only if there is no living female descendant will the legacy go to a man. However, the beneficiary may designate a male if she chooses.
To the knowledge of the Talamasca, the beneficiary of the legacy has never died without designating an heir, and the legacy has never passed to a male child. Rowan Mayfair, the youngest living Mayfair Witch, was designated at birth by her mother Deidre, who was designated at birth by Antha, who was designated by Stella, and so forth and so on.
However, there have been times in the history of the family when the designee has been changed. For example, Marie Claudette designated her first daughter, Claire Marie, and then later changed this designation to Marguerite, her third child, and there is no evidence that Claire Marie ever knew that she was designated, though Marguerite knew she was the heiress long before Marie Claudette's death.
The legacy also provides enormous benefits for the beneficiary's other children (the siblings of the heir) in each generation, the amount for women usually being twice that given to the men. However, no member of the family could inherit from the legacy unless he or she used the name Mayfair publicy and privately. Where laws prohibited the heir from using the name legally, it was nevertheless used customarily, and never legally challenged.
This served to keep alive the name of Mayfair well into the present century. And in numerous instances, members of the family passed the rule on to their descendants along with their fortunes, though nothing legally required them to do so, once they were one step removed from the original legacy.
* Extracted from 'The Witching Hour', pages 474-476