Medicare Parts A, B, C, and D: How They Work

Medicare is designed around four core building blocks: Part A, Part B, Part C, and Part D. While the alphabetical naming system sounds straightforward at first glance, each part covers completely different healthcare services, has its own cost rules, and connects with other parts in specific ways. Understanding how each part functions on its own—and how they fit together into overall coverage pathways—gives you the foundation you need before comparing individual plan options.

Part A: Hospital Insurance

Part A is the hospital insurance portion of Medicare. It is primarily designed to help cover inpatient care when you are formally admitted to a hospital or specialized facility.

Specifically, Medicare Part A covers:

• Inpatient hospital stays: Includes semi-private rooms, general nursing care, meals, and hospital medications during your formal stay.
• Skilled nursing facility (SNF) care: Covers rehabilitation and recovery services following a qualifying three-day inpatient hospital stay.
• Hospice care: Provides palliative and comfort care for individuals with terminal illnesses.
• Home health care: Covers medically necessary, part-time or intermittent skilled nursing and therapy services ordered by a doctor.

For most Americans, Part A is premium-free because they or their spouse paid Medicare payroll taxes while working for at least 40 calendar quarters (approximately 10 years). However, Part A is not free of out-of-pocket costs: beneficiaries are subject to an inpatient hospital deductible per benefit period, as well as daily coinsurance amounts if a hospital stay extends beyond 60 days.

Part B: Medical Insurance

Part B is the medical insurance component of Medicare. Together with Part A, Part B constitutes what the federal government officially calls Original Medicare.

While Part A handles inpatient care, Part B handles outpatient medical care and preventive services, including:

• Doctor visits: Routine appointments, consultations with specialists, and office evaluations.
• Outpatient medical services: Same-day surgical procedures, clinic visits, chemotherapy, dialysis, and emergency room visits.
• Preventive care: Annual wellness exams, flu and pneumonia vaccinations, cardiovascular screenings, mammograms, and bone density scans.
• Diagnostic testing: X-rays, MRIs, CT scans, and clinical laboratory blood work.
• Durable medical equipment (DME): Walkers, wheelchairs, hospital beds, CPAP machines, and blood sugar monitors.

Unlike Part A, almost every beneficiary pays a monthly premium for Part B, which is deducted directly from Social Security checks or billed quarterly. Beneficiaries must also satisfy an annual Part B deductible. Once that deductible is met, Medicare generally pays 80% of approved charges, leaving you responsible for the remaining 20% coinsurance—with no annual cap on your out-of-pocket spending under Original Medicare alone.

Part C: Medicare Advantage

Medicare Part C—widely known as Medicare Advantage—is an alternative way to receive your Medicare Part A and Part B benefits through private insurance companies approved by Medicare.

When you enroll in a Medicare Advantage plan:

• You remain enrolled in the Medicare program and must continue paying your monthly Part B premium.
• The private insurance company administers your Part A and Part B claims instead of the federal government.
• Most Medicare Advantage plans bundle prescription drug coverage (Part D) directly into the plan.
• Plans often include additional benefits not covered by Original Medicare, such as basic dental, vision, hearing aids, or fitness memberships.

However, the way you access medical providers is fundamentally different from Original Medicare. Original Medicare allows you to visit any doctor, specialist, or hospital anywhere in the United States that accepts Medicare, without network limitations or referral requirements. In contrast, most Medicare Advantage plans operate through managed care networks—such as Health Maintenance Organizations (HMOs) or Preferred Provider Organizations (PPOs).

With an HMO, you generally must use in-network doctors and get referrals to see specialists. With a PPO, seeing doctors outside the network typically results in higher out-of-pocket costs. That is why, while low premiums or extra benefits often get the most attention in advertising, the central question for beneficiaries usually comes down to provider access:

"Will this plan let me continue seeing the doctors I use?"

That question can matter just as much.

Part D: Prescription Drug Coverage

Part D is the prescription drug portion of Medicare.

It helps cover prescription medications and is offered through private insurance companies that follow Medicare rules.

You can generally get Part D in one of two ways:

1. A separate Medicare drug plan alongside Original Medicare
2. A Medicare Advantage plan that includes drug coverage

Medicare.gov confirms that most Medicare Advantage plans include Part D, although not every plan works exactly the same way.

The Drug List Matters

Here's where the details really count.

Drug plans use formularies, which are lists of covered prescription drugs. Plans can also have different rules for how drugs are covered and what you pay.

Two plans may look similar until you compare your actual medications.

That is why your prescription list belongs at the center of any drug coverage review.

Before Choosing Drug Coverage

Have these details ready:

• Medication names
• Dosages
• How often you take each drug
• Preferred pharmacies

Then check the plan's formulary and pharmacy information. Guessing isn't a good strategy here.

How the Four Parts Fit Together

Now let's put everything in one picture.

Option 1: Original Medicare

With Original Medicare, you have:

Part A + Part B

You can then add:

Part D for prescription drug coverage.

You can also consider Medigap, which is separate from Medicare Advantage and works alongside Original Medicare to help pay certain out-of-pocket costs.

Option 2: Medicare Advantage

With Medicare Advantage:

Part C provides another way to receive Part A and Part B benefits through a Medicare-approved private plan.

Most Medicare Advantage plans also include Part D drug coverage.

That means you don't normally build your coverage by combining Medicare Advantage with a separate Medigap policy. Medigap is designed to work with Original Medicare, not Medicare Advantage.

Here's the Big Picture

This is the part many people wish someone had explained to them first.

You don't need to memorize every Medicare rule. You need to understand the paths.

What About Medicare Supplement or Medigap?

Medigap is not another Medicare "part."

It's private supplemental insurance that works alongside Original Medicare.

Medigap can help pay certain out-of-pocket costs associated with Part A and Part B, such as coinsurance, copayments, and deductibles, depending on the specific plan. Explore our detailed guide on Medicare Supplement plans to see how plans like Plan G and Plan N operate.

Generally, you need Original Medicare, including both Part A and Part B, to buy a Medigap policy.

And Medigap does not provide prescription drug coverage. If you want drug coverage while using Original Medicare and Medigap, you generally look at a separate Part D plan.

Which Medicare Path Makes Sense for You?

There isn't a universal answer. That's an important point.

Your decision can depend on your doctors, prescriptions, budget, travel habits, and how you want your healthcare costs to work.

Start with a few basic questions:

Do you want broad provider access?

Original Medicare lets you see any doctor or hospital that takes Medicare anywhere in the U.S.

Are your prescriptions a major part of your healthcare spending?

Check the formulary before choosing drug coverage.

Do you care about additional benefits?

Some Medicare Advantage plans may include benefits beyond Original Medicare, but the details vary.

Do you want supplemental coverage alongside Original Medicare?

That is where Medigap comes into the conversation.

The point isn't to find one option that sounds best in a headline. The point is to understand what you're actually choosing.

Don't Forget the Enrollment Timeline

Knowing the four parts is only half the job. Timing matters too.

Most people have a seven-month Initial Enrollment Period around their 65th birthday. Depending on your circumstances, other enrollment periods may apply. For a full breakdown of these dates, review our turning 65 enrollment checklist.

There is also a Medicare Open Enrollment period each year from October 15 through December 7. During this period, eligible beneficiaries can make certain changes to Medicare health and drug coverage. Learn more in our guide on when you can switch plans.

If you're still working at 65, employer coverage can affect your Medicare timing. If you or your spouse have group health plan coverage based on current employment, you may be able to delay Part B and use a Special Enrollment Period later. Confirm how your employer coverage works with Medicare before delaying enrollment.

Don't make a timing decision based on what a friend did. Your situation may be different.

A Simple Medicare Checklist

Before you make a coverage decision, write down these seven things:

This gives you a much better starting point than trying to compare every plan at once.

Common Questions People Ask

Conclusion

Understanding Medicare Parts A, B, C, and D gives you the foundation you need before comparing coverage. Part A handles certain hospital and inpatient care. Part B handles many outpatient and medical services. Part C gives you another way to receive Part A and Part B benefits through a Medicare-approved private plan. Part D helps with prescription drug costs.

Once those four pieces are clear, the next step is looking at how they fit with your doctors, prescriptions, budget, and lifestyle.

Don't choose based on a catchy ad. Don't assume your neighbor's plan will work for you. Start with what you actually need, then compare the options available in your area.

Need Help Making Sense of Your Options?

Start With Clarity65

Medicare gets easier once someone explains the pieces in plain English.

Clarity65's licensed agents help people understand the differences between Original Medicare, Medicare Advantage, Medigap, and prescription drug coverage. You can also review our detailed guides on Medicare Advantage, Medicare Supplement, and Part D drug plans to learn how each option works.

You don't need to walk into a Medicare decision knowing all the answers. You just need to know what questions to ask.

Raul Gonzalez

Licensed Agent

Independent licensed Medicare advisor dedicated to plain-English guidance and personalized plan comparisons across leading national carriers.

Clarity65 is not connected with or endorsed by the U.S. government or the federal Medicare program.