A 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 can help pay for some of the costs that Original Medicare doesn't, like copayments, coinsurance, and deductibles. 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. Cost is usually the only difference between Medigap policies with the same plan letter sold by different insurance companies.

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.

You must have Part A and Part B to buy a Medigap policy. 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 (PDPs), employer/union group health coverage (GHP), Medicaid, Department of Veterans Affairs (VA) benefits, or TRICARE.

Also, it may be illegal for the insurance company to sell you a Medigap policy if you already have Medicaid or a Medicare Advantage Plan. Medigap policies cover only one person.

If you and your spouse both want Medigap coverage, you’ll need to buy separate Medigap policies. You still pay the monthly Part B premium.