Binyan av mikatuv echad [#3a Ishmael]
This principle may be translated from a General Principle Found in One Verse. This allows the inference of a rule [herein called the Av or father], from a single passage of Scripture, for all similar passages or cases. For instance, a binyan av tells us that just as the hand is used to slaughter and pour the blood of a slaughtered animal on to the ground, so must the hand be used to cover the blood with dirt – not the foot. Leviticus 17:13 reads, He shall pour out the blood thereof and cover it with dust. R’Joseph derives the concept of respectful treatment of all precepts of Torah. By observing that blood is the Av, one must take respectful care of the slaughter, so we must respectfully dispose of the blood. In like manner, so we must respectfully carry out all the precepts of Torah [cf. b.Shabbat 22a]. We’re given a leniency in Torah that states we’re able to prepare food on the day after the Pesach – the first day of Matzot. A binyan av permits us to extend the rule to all the other holidays [except Yom Kippur] as well.
Other examples of the binyan av is found in Exodus 21:26-27, where the loss of an eye or a tooth is grounds for redemption from slavery. The Rabbis conclude that the loss of any body part is grounds for redemption. In Deuteronomy 24:6 where the hand mill and upper millstone are exempt from pledges; because this would be taking a life in pledge, we determine that everything used in the preparation of food is likewise exempt from pledges.
In b.Yevamot 54b, a binyan av is brought forth on the prohibition to marry one’s maternal half sister. Since a man cannot marry his father’s sister, he cannot marry his father’s maternal half sister either.