A Medicare Supplement, or Medigap policy is an insurance policy that helps fill "gaps" in Original Medicare and is sold by private companies. Original Medicare pays for much, but not all, of the cost for covered health care services and supplies. Medigap policies are sold by private companies, and can help pay for some of the costs that Original Medicare doesn't, like copayments, coinsurance, and deductibles.

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Some Medigap policies also cover certain benefits Original Medicare doesn’t cover, like emergency foreign travel expenses. Medigap policies don’t cover your share of the costs under other types of health coverage, including Medicare Advantage Plans, stand-alone Medicare drug plans, employer/union group health coverage, Medicaid, or TRICARE.

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If you have Original Medicare and a Medigap policy, Medicare will pay its share of the Medicare-approved amounts for covered health care costs. Then, your Medigap policy pays its share. Medicare doesn't pay any of the costs of buying a Medigap policy.

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A Medigap policy is different from a Medicare Advantage Plan because those plans are another way to get your Part A and Part B benefits, while a Medigap policy only helps pay for the costs that Original Medicare doesn't cover. Insurance companies generally can’t sell you a Medigap policy if you have coverage through a Medicare Advantage Plan or Medicaid.

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All Medigap policies must follow federal and state laws designed to protect you, and policies must be clearly identified as “Medicare Supplement Insurance.” Medigap policies are standardized, and in most states are named by letters, Plans A–N. Each standardized Medigap policy under the same plan letter must offer the same basic benefits, no matter which insurance company sells it.