Part A, B, C, D, and Medigap: What Each Part of Medicare Covers

Medicare isn't one single benefit — it's made up of separate parts, each covering something different, and understanding which part does what is the foundation for every other Medicare decision. Part A covers hospital care. Part B covers medical care outside the hospital. Part C, known as Medicare Advantage, is a private-plan alternative that bundles the others together. Part D covers prescription drugs. And Medigap, while not technically a "Part," is a supplemental policy people commonly add alongside Original Medicare. Here is what each one actually does.

Part A: hospital insurance

Part A is often called hospital insurance because it covers inpatient care — care you receive while formally admitted to a hospital, as well as skilled nursing facility care after a qualifying hospital stay, hospice care, and some home health services. If you're admitted to a hospital overnight for treatment, Part A is generally the coverage handling that stay.

Most people don't pay a monthly premium for Part A. Eligibility for premium-free Part A is based on work history — if you or your spouse paid Medicare payroll taxes for enough years, generally around a decade of qualifying work, Part A comes without a monthly premium. Part A still has its own deductible and cost-sharing rules for hospital stays, separate from any premium. See Medicare.gov's explanation of what Part A covers for the full list.

Part B: medical insurance

Part B covers medical care outside a hospital admission: doctor visits, outpatient procedures, preventive services like screenings and vaccines, durable medical equipment (things like wheelchairs or oxygen equipment), and outpatient mental health services, among other things. If Part A is "care while admitted," Part B is closer to "everything else your doctor does for you."

Almost everyone pays a monthly premium for Part B, set annually, plus an annual deductible and ongoing coinsurance on covered services. Higher earners may pay an additional income-related amount on top of the standard premium. Together, Part A and Part B make up what's called Original Medicare — the traditional, federally administered version of the program. Medicare.gov's Part B coverage page has the full breakdown of what's included.

Part C: Medicare Advantage

Part C, more commonly called Medicare Advantage, is not a separate kind of coverage in the way Part A and Part B are — it's a different way of getting your Part A and Part B benefits, delivered through a private insurance company instead of directly through the government. Medicare Advantage plans are required to cover everything Original Medicare covers, and most also bundle in Part D prescription drug coverage plus extra benefits like dental or vision that Original Medicare doesn't offer at all.

The trade-off is that Medicare Advantage plans generally use a network of doctors and hospitals, and may require referrals to see specialists or prior authorization before certain services. In exchange, plans are required to include an annual cap on your out-of-pocket spending, something Original Medicare alone does not have. For a full comparison of this trade-off, see Medicare Advantage versus Medigap.

Part D: prescription drug coverage

Part D covers prescription medications. It's available in two forms: as a stand-alone Part D plan you add on top of Original Medicare, or bundled into a Medicare Advantage plan (most, but not all, Advantage plans include it). Each Part D plan maintains its own formulary — the specific list of drugs it covers and the cost tier each one is placed on — so two different Part D plans can charge very different amounts for the exact same medication.

If you don't have Part D coverage and don't have other creditable prescription drug coverage, such as through a former employer, you can face a late-enrollment penalty added to your Part D premium if you sign up later. Medicare.gov's Part D overview explains how the drug benefit and its enrollment rules work.

Medigap: supplemental coverage, not technically a "Part"

Medigap, also called Medicare Supplement insurance, isn't one of the lettered parts of Medicare, but it's commonly grouped alongside them because of how central it is to how many people structure their coverage. Medigap is a policy sold by private insurers that works alongside Original Medicare (Part A and Part B) to help cover some of the costs Original Medicare leaves you responsible for, such as coinsurance and, depending on the specific plan letter, deductibles.

Medigap only works with Original Medicare — you cannot use a Medigap policy alongside a Medicare Advantage plan. It's priced separately from Part A, Part B, and Part D, meaning someone using Original Medicare with a Medigap policy and a stand-alone Part D plan is typically paying three separate premiums rather than one bundled one. In exchange, this combination generally offers the widest provider access, since Original Medicare doesn't use a network the way Medicare Advantage does. Medicare.gov's Medigap overview explains how supplement policies work and when you can enroll with guaranteed acceptance.

How the parts fit together

Most people end up on one of two structures:

1. Original Medicare (Part A + Part B) + stand-alone Part D + optional Medigap. Broadest provider access, no network to check, but generally the most separate premiums to manage and no built-in out-of-pocket cap unless a Medigap policy provides one.
2. Medicare Advantage (Part C), usually including Part D. Typically one plan, often a lower combined premium, extra benefits, a required annual out-of-pocket maximum — in exchange for a provider network and plan-specific rules.

Neither structure is the "normal" one and neither is a mistake. They're two different ways of assembling the same underlying coverage, and the right one depends on your doctors, your health, your budget, and how much you value open provider access versus predictable costs and extra benefits.

The bottom line

Part A covers hospital care. Part B covers medical care outside the hospital. Part C, Medicare Advantage, is a private-plan bundle of the two, usually with Part D and extra benefits added, in exchange for network rules. Part D covers prescription drugs, either stand-alone or bundled into an Advantage plan. Medigap is a supplemental policy that works alongside Original Medicare to reduce your cost-sharing exposure, though it's not one of the numbered or lettered parts itself. Knowing what each piece actually covers is the starting point for every other decision about how to structure your Medicare coverage.

For the official description of each part of Medicare, see Medicare.gov's "What's Medicare" overview and CMS's Medicare program information.