The Heter of 100 Rabbis:[1]

Many Rishonim[2] and Poskim[3] record that one can be released from the ban of Rabbeinu Gershom and marry a second wife, if he receives permission to do so from 100 Rabbis[4] from three different communities in three different lands[5], based on reasons that they agree to be clear to justify the narrowing of the second wife. [From the wording of many of the Rishonim, it is implied that this allowance was instituted years later, after the ban was placed.[6] However, from other Rishonim it is evident that this allowance was instituted by Rabbeinu Gershom himself.[7]]

Kesuba money:[8] Even in the case that an allowance is given, the husband must leave the sum of the Kesuba with a trustworthy person in the event that his first wife agrees to get divorced.

Is the Heter of 100 Rabbis needed today: Some Poskim[9] rule that in today’s times, it is no longer necessary to get a Heter from 100 Rabbis to marry a second wife, in those cases of justifiable reason. Other Poskim[10], however, rule that it is required even today, and so is the practical ruling.

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[1] See Beis Shmuel 1:23; Encyclopedia Talmudit Vol. 17 Erech Cherem Derabbeinu Gershom sec. 17 pp. 447-452 footnotes 814-874

[2] Orchos Chaim 3 “Takanos Rabbeinu Gershom” 17; Maharam Padvah 14; Kol Bo 116, brought in Darkei Moshe 1:11

[3] Beis Shmuel 1:23; 119:8; Chelkas Mechokeik 1:16; 119:7; Darkei Moshe 1:9

[4] See Encyclopedia ibid p. 449 footnotes 836-843 for the exact definition of a Rabbi for this matter. They do not have to be Rabbis who lead communities and if they are fit to give rulings then they are valid. [Noda Beyehuda Kama E.H. 3; Chasam Sofer 1 E.H. 4]

[5] See Chasam Sofer ibid and Encyclopedia ibid for the definition of this matter

[6] Tzemach Tzedek [Kadmon] 67

[7] Bach 93; E.H. 1; Hagahos Mordechai Yevamos 108; Chasam Sofer E.H. 1:3; See Encyclopedia ibid footnotes 819-820

[8] Beis Shmuel 1:23; Maharam Padvah 14

[9] Darkei Moshe 1:10, brought in Beis Shmuel 1:23; Tosafos Beitza 5a; Hagahos Ashri Beitza 3; This requirement is omitted from the Rama in his Shulchan Aruch; See Encyclopedia ibid footnote 830

[10] Beis Shmuel 1:23 and 119:8 in name of Bach that so is a Kabbalah from previous generations; Chelkas Mechokeik 1:16 and 119:7; See Pischeiy Teshuvah 1:16; See Encyclopedia ibid footnote 831