From the Title On Betrothals and Marriage

Chapter I. (The same, to the Abbot of Saint Alban). It is held from the canons that no one be united in marriage to the one whom he had previously polluted by adultery, and especially to that one to whom he had given faith while his wife was living, or who plotted for the death of the wife.

Chapter III. (The same, to the Bishop of Vigilia). If someone has betrothed a girl below the twelfth year, before both have arrived at years apt for marriage, they cannot withdraw from the nuptial pact and fly to another marriage.

Chapter IV. (The same, ibid.). No blood-relative of another can ever unite in marriage the other's betrothed, if she is near marriageable age.

Chapter VIII. (The same, in a letter to the Bishop of Salerno). If legitimate consent ... intervenes concerning the present in such wise that one receives the other as his own by mutual consent in expressed words, each saying: I receive you as mine, and I you as mine, whether there is an oath or not, it is not licit for the woman to marry another; and if she shall have married and carnal copula shall have followed, she ought to be separated from him and compelled by ecclesiastical constraint to return to the first, although others think otherwise, and at another time it was also judged otherwise by certain of our predecessors.

Cf. Thesis XVIII, V.