The nights over rule is the IRS method to determine which is the custodial parent when the child's parents do not live together. Simply put, the parent at whose home the child slept over half the nights in a year is determined to be the custodial parent. This overrides anything in the divorcee decree, but the custodial parent may relinquish the right by executing Form 8332. Custody provides a number of tax benefits, but not all can be relinquished with an 8332. For more detail see my blog www.taxconnections.com/taxblog/who-gets-the-kids/.