Bava Batra 131B

ממאי דבחד בי דינא איתקון? דלמא בתרי בי דינא איתקון!

From where do you conclude that both clauses, the one for the benefit of the sons and the one for the benefit of the daughters, were instituted by one court and are therefore each read in light of the other? Perhaps they were instituted by two different courts and are unrelated to each other. Accordingly, the wording of the gift and the wording of the inheritance are not part of the same statement.

לא סלקא דעתך, דקתני רישא: זה מדרש דרש רבי אלעזר בן עזריה לפני חכמים בכרם ביבנה: הבנים יירשו והבנות יזונו; מה הבנים אינן יורשין אלא לאחר מיתת אביהם, אף בנות לא יזונו אלא לאחר מיתת אביהן.

The Talmud answers: This possibility should not enter your mind, as the former clause of that Mishnah teaches (Ketubot 49a): A father is not obligated to provide his daughter’s sustenance. This interpretation was interpreted by R' Elazar ben Azarya before the rabbis of the vineyard in Yavne: Since the rabbis instituted that after the father’s death, the sons inherit the sum of money specified in their mother’s marriage contract, and the daughters are sustained from their father’s estate, these two halakhot are equated: Just as the sons inherit only after their father’s death, not during his lifetime, so too, the daughters are sustained from their father’s property only after their father’s death.

אי אמרת בשלמא בחד בי דינא איתקון, היינו דילפינן תקנה מתקנה. אלא אי אמרת בתרי בי דינא איתקון, היכי ילפינן תקנה מתקנה?

The Talmud explains: Granted, if you say that the two clauses were instituted by one court, this is the reason that we derive the halakhot of one ordinance from the halakhot of the other ordinance, under the assumption that a court institutes ordinances in a consistent manner. But if you say that they were instituted by two distinct courts operating in different periods, how can we derive the halakhot of one ordinance from the halakhot of the other ordinance?