The postnup is a relatively new animal. Typically, it's signed after one member of the marriage strays, a la Bill Clinton, yet wants to remain in the marriage. Often, the straying party agrees to the postnup as an inducement for their spouse to take them back. Staying married with a postnup is cheaper than a divorce, assuming that the one who strays is the richer of the two. As with a prenup, those considering postnups should have separate counsel, make full disclosure of all property and avoid anything that might be construed as duress.