Maseches Kesubos, Chapter 4, Mishnah 10. Before us is another one of the conditions of the kesubah, a condition that remains in force even if the husband does not write it into the document, and it concerns the concept of 'kesubas benin dichrin' (the kesubah of male children).
The essence of the law:
A man who had two wives, and from them two sets of sons - although ordinarily all the sons inherit the estate equally, they do not inherit their mothers' kesubah equally. The rest of the estate they divide equally, but the sum of each wife's kesubah is divided only among her own sons.
The wording of the Mishnah:
"Lo kasav lah" - if the husband did not write for his wife the following text.
"Benin dichrin di yehevun lich minai" - male children that you will have from me, that you will bear.
"Inun yirsun kesaf kesubasich" - they will inherit the money of your kesubah.
"Yeser al chulakhon im achuhon" - beyond the portion they would receive together with their brothers, meaning that this sum is not divided equally like the rest of his property.
The case under discussion is where the wife died during her husband's lifetime, or both wives died during his lifetime, and the husband inherited their kesubah. We do not say that the kesubah money becomes part of the rest of his property and the sons divide it equally; rather, each group of sons inherits its own mother's kesubah.
"Chayav, shehu tenai beis din":
Even if the husband did not write this condition into the document - "chayav" - he is still bound by the law of kesubas benin dichrin, "shehu tenai beis din" - for it is a condition of beis din, a condition established by beis din, and its force does not depend on its being written.
In summary: In this Mishnah we learned the condition of kesubas benin dichrin: each wife's sons alone inherit the money of their mother's kesubah, beyond their equal share in the rest of the estate, which is divided among all the sons. This condition applies even when it was not written in the document, because it is a condition of beis din.