A rule used when a person is covered by more than one insurance plan, telling the insurers which plan pays first, which pays second, and how much each must cover so the same bill is not paid twice.

This comes up fast after a crash, work injury, or medical emergency, especially when someone has health coverage through a job, a spouse, Medicare, Medicaid, or an auto policy with medical payments coverage. One insurer becomes the primary payer and handles the bill first. The secondary payer may cover some or all of what is left, depending on its policy terms. If the plans disagree, payment can stall while treatment bills pile up. That delay can hurt credit, interrupt care, or push a provider to send an account to collections.

For an injury claim, coordination of benefits can directly affect what money is still available from a settlement and whether an insurer later demands reimbursement through a lien or subrogation claim. Federal rules matter here: the Medicare Secondary Payer Act, 42 U.S.C. § 1395y(b), requires Medicare to pay second in many situations and gives the government a right to recover conditional payments. Miss a notice, give the wrong insurer first billing, or settle without checking repayment rights, and money can be lost quickly. Most claims settle before trial, so these payment issues usually have to be sorted out early, not at the end.