A postnuptial agreement is "an important and effective tool in Illinois and should not be forgotten by practitioners," writes Chicago lawyers Joshua S. Singewald and Kellie Rose Bylica in the September ISBA Family Law Section newsletter.

As in the case of any contract, a postnup must be entered into voluntarily by the parties, and the familiar contract-law protections against coercion and mistake apply, Singewald and Bylica note. And Illinois law "require[s] a full disclosure of each party's assets and liabilities for every postnuptial agreement," allowing each "to make an informed decision."

So when would a married couple use one? "[I]f one spouse intends to start a business during the marriage, the business-owning spouse may want to protect his or her business in the event of divorce," Singewald and Bylica write. "Similarly…a spouse [who] makes a substantial non-marital contribution to a marital asset," such as making the down payment on a house, might want to assure reimbursement, they write. Postnups also enable the parties to allocate debt, deal with bequests to children of previous marriages, and even "[r]evise a prenuptial agreement based on current/changed circumstances."

Learn more about postnups in the September Family Law newsletter.