Medigap (also Medicare supplement insurance or Medicare supplemental insurance) refers to various private health insurance plans sold to supplement Medicare in the United States. Medigap insurance provides coverage for many of the co-pays and some of the co-insurance related to Medicare-covered hospital, skilled nursing facility, home health care, ambulance, durable medical equipment, and doctor charges. Medigap's name is derived from the notion that it exists to cover the difference or "gap" between the expenses reimbursed to providers by Medicare Parts A and B for the preceding named services and the total amount allowed to be charged for those services by the United States Centers for Medicare and Medicaid Services (CMS). As of 2006, 18% of Medicare beneficiaries were covered by a Medigap policy. Public-option Part C Medicare Advantage health plans and private employee retiree insurance provides a similar supplemental role for almost all other Medicare beneficiaries not dual eligible for Medicaid.