FOURTH LATERAN COUNCIL, 1215, c. 51 (=X 4.3.3), in G. Alberigo et al., Conciliorum oecumenicorum decreta 258 (3d. ed. 1972)
"Although the prohibition of the conjugal bond has been revoked in the last three degrees, in other degrees we want it strictly observed.(1) Wherefore, following in the footsteps of our predecessors, we strictly forbid clandestine marriages, also forbidding any priest from presuming to participate in them. Therefore, extending generally to all places the custom of some places, we decree that when marriages are to be contracted, they shall be proclaimed publicly in the church by the priests, and that an appropriate time be set within which anyone who wishes to and can may bring forward a lawful impediment. Regardless of whether this happens, the same priests shall investigate whether any impediment exists. When a probable conjecture appears against the joining, let the contract expressly be forbidden until it can clearly be established by documentary evidence what ought to be done about it.
"If anyone presumes to enter into this kind of clandestine or interdicted marriage in the forbidden degrees of kinship, even if unknowingly, the progeny born of such marriage shall be deemed illegitimate, having no assistance from the ignorance of their parents, even though those who so contract seem not to be privy to the knowledge or rather pretend ignorance. Similarly, off-spring shall be deemed illegitimate if both parents, knowing of a lawful impediment, despite all interdict, presume to contract in the face of the church.
"Clearly any parish priest who fails to prohibit such unions or any regular priest [i.e., a member of a religious order] who presumes to becomes involved with them ought to be suspended from office for three years and should be more severely punished if the gravity of the fault demands it. And also let a fitting penance be imposed on those who enter into such marriages even if in a permitted degree [of kinship]. Moreover, if anyone maliciously interposes an impediment to a lawful joining, let him not escape ecclesiastical sanction."
1. Lateran IV, c. 50, had reduced the degrees of kinship within which marriage was prohibited from seven to four, i.e. previously marriage had been prohibited among sixth cousins and anyone more closely related; the Council reduced it to third cousins.
Note to the student: The prohibition of clandestine marriage and the concomitant requirement of the publication of banns was repeated in various forms in over thirty pieces of English conciliar legislation and episcopal decrees between 1200 and 1342. Several of these were collected and glossed in his Provinciale by the great English canonist William Lyndwood, who tells us that he finished his work on the Vigil of Pentecost, 1430.