This is a comparatively brief sugya, Kiddushin 50b—51a. The meaning of the central term is: where the law is insistent that a certain effect - B - cannot follow another effect – A – then it implies that A and B cannot take effect even when they occur simultaneously. Since the taking effect of A frustrates the taking effect of B and the taking effect of B frustrates the taking effect of A, this frustration operates not only when one follows the others but even when both occur together, even though if each were to occur on its own it would take effect.

The sugya is here appended to a Mishnah which reads: ‘If a man betroths a woman and her daughter or a woman and her sister simultaneously, they are not betrothed’ (i.e. the betrothal is invalid and no get, ‘bill of divorce’, is required before either can marry another). A man cannot betroth his wife's daughter, mother or sister and if he did the act of betrothal is invalid (Leviticus 18: 17–18). This is stated explicitly in the Torah. But the Mishnah goes further to rule that if he betroths a woman and her daughter or two sisters, the betrothal to take effect simultaneously, it is still invalid. The Talmud asks how do we know this law, to which the fourth-century Babylonian Amora, Rmai bar Hama, replies, it is because Scripture states: ‘Neither shalt thou take a wife to her sister, to vex her’ (Leviticus 18: 18).