The only real difference concerns timing. A postnuptial agreement is made after the marriage has already commenced. It covers similar topics to a prenuptial agreement. If you sign a postnuptial agreement, you may give up some of the rights you acquired when you became married. Note that both prenuptial and postnuptial agreements are binding documents. However, circumstances do change between married couples and agreements can be changed if both spouses agree. Indeed, after ten years or so of marriage, it might be considered wise to review and update your agreement.