Yevamos 56
b"In what ways are inferior relations effective for Levirate marriage? \\u2022 Does Yibum require witnesses? \\u2022 Why is Yevamos before Kesubos? \\u2022 Would Jewish courts coerce a recalcitrant man to marry? \\u2022 A woman's infidelity renders her prohibited to her husband \\u2022 This prohibition of defilement requires the testimony of two witnesses to warrant administering lashes to the husband"