/summa/XP/XPII/XP_Q43:
In the next place we must consider matrimony absolutely; and here we must treat (1) of the betrothal; (2) of the nature of matrimony; (3) of its efficient cause, namely the consent; (4) of its blessings; (5) of the impediments thereto; (6) of second marriages; (7) of certain things annexed to marriage.
Under the first head there are three points of inquiry:
(1) What is the betrothal?
(2) Who can contract a betrothal?
(3) Whether a betrothal can be canceled?