Reference for Yevamot 62:15

ונייבם לחדא וניחלוץ לחדא גזירה שמא יאמרו בית אחד מקצתו בנוי

MUST PERFORM <i>HALIZAH</i><span class="x" onmousemove="('comment',' With the surviving brother. ');">³⁹</span> BUT MAY NOT BE TAKEN IN LEVIRATE MARRIAGE; FOR IT IS SAID. AND ONE OF THEM DIED [ETC.] HER HUSBAND'S BROTHER SHALL GO IN UNTO HER.<span class="x" onmousemove="('comment',' Deut. xxv, 5. ');">⁴⁰</span> ONLY SHE WHO IS BOUND TO ONE LEVIR<span class="x" onmousemove="('comment',' Is to be married by him. ');">⁴¹</span> BUT NOT SHE WHO IS BOUND TO TWO LEVIRS.<span class="x" onmousemove="('comment',' The first to whom she was bound by the levirate tie and the second to whom she is bound by the ma'amar. A ma'amar of a levir, unlike his levirate marriage. cannot sever the bond between the widow and her deceased husband-the levirate tie. ');">⁴²</span> R. SIMEON SAID: HE MAY TAKE IN LEVIRATE MARRIAGE WHICHEVER OF THESE HE WISHES<span class="x" onmousemove="('comment',' v. supra 19a. If the ma'amar has the validity of marriage, the surviving levir is marrying his second brother's wife, and if a ma'amar is invalid he is marrying either the wife of his first brother or the wife of the second. ');">⁴³</span> AND THEN PARTICIPATE IN THE <i>HALIZAH</i> WITH THE OTHER.<span class="x" onmousemove="('comment',' The levirate marriage of the one cannot exempt the other from the halizah, since it is possible that a ma'amar is invalid and the two widows are consequently of different brothers. He may not marry the two, since a ma'amar may be valid and he would thus be marrying two widows of the same brother. ');">⁴⁴</span> <b><i>GEMARA</i></b>. If, however, the levirate bond with two levirs<span class="x" onmousemove="('comment',' According to the Rabbis of our Mishnah. ');">⁴⁵</span> is Pentateuchal,<span class="x" onmousemove="('comment',' Since they forbade the levirate marriage in such a case. ');">⁴⁶</span> even <i>halizah</i> should not be required! — But it<span class="x" onmousemove="('comment',' The levirate bond with two levirs. ');">⁴⁷</span> is only Rabbinical,<span class="x" onmousemove="('comment',' Pentateuchally a ma'amar is not binding. and the two widows consequently are of two different brothers and may both be married. ');">⁴⁸</span> a preventive measure having been enacted against the possible assumption that two sisters-in-law coming from the same house<span class="x" onmousemove="('comment',' l.e., widows of the same brother. ');">⁴⁹</span> may both be taken in levirate marriage. Then let one be taken in levirate marriage and the other be required to perform <i>halizah</i>! — A preventive measure has been enacted against the possible assumption that one house was partially built

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