Ketubot, Chapter Four, Mishnah Ten
This mishnah refers to a guarantee that a husband must give to his wife that her ketubah (in this case ketubah refers to money she brought into the marriage as a dowry) will be inherited by her sons. This will be explained below. Even if the husband does not write this guarantee in the ketubah, the sons still receive the inheritance.
If he did not write for her, The male children that will be born from our marriage shall inherit the money of your ketubah over and above their shares with their brothers, he is nevertheless liable, because [this clause] is a condition laid down by the court.
The ketubah in this mishnah refers to money that the wife brought into the marriage as a dowry. When a wife dies her husband inherits this money and it becomes officially his property and not just his to use, as it was during the marriage. Written into the ketubah document is a guarantee that this wifes male children will inherit this amount over and above their inheritance from the remainder of the fathers estate. For instance, let us say that a woman marries a man. She brings into the estate property in the value of 1000 zuz, has five male children and then she dies. The husband inherits this money and then remarries, this time a wealthy woman, who brings 10,000 zuz into the marriage and then has one son and dies. When the husband dies, he leaves an estate of 15,000 zuz. The one son of the rich woman inherits 10,000 zuz off the top. This is his mothers ketubah and although it became his fathers property, he inherits that amount separately from the general inheritance (as long as it still exists). The five sons from the first marriage split the 200 zuz from their mothers ketubah, each son taking 200 zuz. There are now 4,000 zuz left in the estate and each son takes an equal portion, except for the oldest son who takes a double portion.
The purpose of this ketubah clause is so that a rich father-in-law will write a handsome dowry for his daughter. A father-in-law might fear that his son-in-law will have children from another wife and these children will inherit property that he wanted to see his grandsons have. Through this guarantee a father-in-law can be more certain that his property will stay with his blood relatives.