When someone is behind in child support or alimony (a/k/a spousal maintenance) payments, the back amount owed is called arrears. The most common way for someone to pay arrears is to add twenty percent (20%) to any ongoing support payments. That way, the current month is paid up completely and there is the 20% overage to apply to arrears. This may result in a very long time frame for satisfying arrears, if the amount owed high.
If there are arrears owing when child support ends (because the child or children are no longer minors), they still must be paid. Those arrears will be paid in one of a variety of ways. One, the obligor may keep paying the 20% overage until the arrears are fully satisfied. Two, a judgment may be entered against the obligor, and the arrears are satisfied by garnishing wages, levying bank accounts or intercepting tax refunds. Three, there can be a negotiated settlement and schedule of payment.