* = This is an Ervah d'Oraisa and not just a Sheniyah
( ) = The Chachamim did not prohibit this case as a Sheniyah

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FOOTNOTES:

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(1) There is a general rule that any Ervah that is Asur mid'Oraisa in one generation is also Asur mid'Rabanan as a Sheniyah in subsequent generations and in previous generations. Any Ervah that is only Asur mid'Rabanan even in the closest generation (due to an Ervah mid'Oraisa to which it is somewhat similar) is not Asur in subsequent or preceding generations -- that is, it has a "Hefsek" (RASHI 21a DH v'Eshes Achi ha'Av. There are several exceptions to this rule -- see footnotes 2 and 3.)

(2) In certain cases, the Rabanan prohibited a woman because she is often confused with another woman that they prohibited as a Sheniyah:

1. Even though his mother's mother is only Asur to him mid'Rabanan, the Chachamim nevertheless decreed that his father's mother is also Asur in order that one not mistakenly permit his mother's mother. This is not considered a Gezeirah l'Gezeirah, "because both are called 'grandmother'" and it is thus very likely that the two will be confused with each other.

2. Similarly, the Chachamim prohibited the wife of his mother's father (#4) because of the wife of his father's father, even though the latter is only Asur mid'Rabanan, "because both are called 'grandfather'" and are likely to become confused with each other.

3. Similarly, the wife of his mother's brother from the same mother (#9) is prohibited because of the wife of his mother's brother from the same father which itself is only Asur mid'Rabanan, "because both are called 'aunts'."

4. Similarly, the Chachamim prohibited the daughter-in-law of his daughter because of the daughter-in-law of his son, because both women's husbands are "grandsons" and therefore they are easy to confuse with each other.

(3) Even though there is no generation in which this relationship is Asur mid'Oraisa, the Chachamim prohibited her for all generations because in the first generation it is very easy to confuse her with an Ervah d'Oraisa (see footnote #2). (RASHI)

(4) This is the reasoning of Ze'iri, who adds that the wife of his mother's father has a Hefsek and is not Asur in other generations. (Rav argues and equates her with the mother of his father, who is Asur without a Hefsek, as above in footnote #3).

(5) The Gemara poses as a question whether the Chachamim prohibited the other generations of this Sheniyah, and concludes that they did not.

(6) TOSFOS (DH Lo Asru) adds that it is fitting to prohibit her for another reason -- she is included in the rule that "in any case of a female who is Asur because of Ervah, if that female were a male, his wife would be Asur mid'Rabanan as a Sheniyah." (See Chart #7)

(7) Rav Mesharsheya poses the question of whether these women were prohibited as Sheniyos. The Gemara's conclusion is that they are permitted.

(8) This Halachah is not explained in the Gemara. (In any case, the Gemara concludes that these women are permitted, as we wrote above in footnote #7). Apparently, according to the Havah Amina that they are prohibited, it would also be fitting to prohibit them without a Hefsek, since their first generation is Asur mid'Oraisa.

(9) TOSFOS (DH Eshes) equates his mother's mother's sister to his father's father's sister.

(10) The Gemara does not mention, in the list of Sheniyos of Rebbi Chiya, the status of the daughter of the daughter of his daughter, or the status of the daughter of the daughter of his wife's daughter; see Insights.

(11) The Gemara does not answer this question. The RAMBAM rules leniently, because "Safek d'Rabanan l'Kula."

(12) The Gemara explains that Rav Yehudah bar Shila stated this principle only with regard to relatives who are related to the man himself, and not with regard to relatives who are related to him through his wife (such as #4-9, and #12), since they are his relatives only by way of two marriages.

(13) According to TOSFOS 21b DH Lo Asru, this is also the reason for the prohibition: since the female relative is an Ervah, the Chachamim prohibited the male's wife as a Sheniyah.

(14) This applies whether they share the same mother or the same father. (From here the Gemara proves that the wife of the brother of his mother from the same mother is prohibited as a Sheniyah).

(15) The wife of the brother of his father, from the same father is Asur mid'Oraisa, and from the same mother -- is prohibited as a Sheniyah.