An agreement made before marriage that defines each partner's interest in the other partner's property is a collateral promise .

Collateral Promise

A secondary promise that is ancillary (subsidiary) to a principal transaction or primary contractual relationship, such as a promise made by one person to pay the debts of another if the latter fails to perform. A collateral promise normally must be in writing to be enforceable.

Before Marriage

Refers to the period or status of individuals' lives before entering into a legally recognized marital union.