Medicare Parts A, B, C, and D: core concepts and coverage basics

I didn’t wake up one day magically understanding Medicare. It crept in as a practical question from a parent, a neighbor, and—if I’m honest—a future version of me. Four letters seemed to run the whole show: A, B, C, D. At first I treated them like a secret code only insurance pros could crack. But after a few evenings with the official handbook, several calls with my state’s counseling program, and a lot of note-taking, the fog lifted. What follows is my personal field guide—how the pieces fit together, what choices actually matter, and the quiet details that prevent expensive surprises later.

The big picture that finally made sense

Here’s the simple backbone: Original Medicare is Parts A and B. You can keep it as is and add optional Part D and (often) a Medigap policy, or you can choose Medicare Advantage (Part C), which bundles A and B and usually D into one private plan with its own network and rules. That’s the whole tree; every branch is just a detail hanging from it. The moment I kept those two pathways straight—Original Medicare vs. Advantage—everything else snapped into place. For clarity, the official handbook (“Medicare & You”) is the gold-standard overview and worth bookmarking; you can find it on Medicare.gov.

• Original Medicare = Part A (hospital) + Part B (medical/outpatient) managed by the federal government.
• Medicare Advantage (Part C) = private plan alternative that must cover A and B services and usually adds extras like vision/dental; it follows plan rules and networks.
• Part D = prescription drug coverage that you can add to Original Medicare (or receive as part of many Advantage plans).

Each route has trade-offs: Original Medicare has broad acceptance and predictable coverage rules but no out-of-pocket maximum; many people pair it with a Medigap policy and a standalone Part D plan. Advantage plans cap your in-network spending for Part A/B services and may include extras, but you’ll live with networks, referrals, and prior authorization rules. None of this is inherently “better”—it’s about fit.

What Part A actually pays for in real life

Part A is hospital insurance. It generally covers inpatient hospital stays, limited skilled nursing facility care after a qualifying inpatient stay, some home health, and hospice. For most people who worked and paid Medicare taxes for at least 10 years (40 quarters), Part A has no monthly premium. That was a relief to learn. Costs like deductibles and coinsurance can apply and change annually, so I treat the official cost page as my north star on numbers: the costs section at Medicare.gov keeps the current amounts.

• Hospital benefit periods can reset, so a single calendar year can include more than one deductible if benefit periods are separated.
• Skilled nursing facility coverage depends on qualifying inpatient days and is limited; it’s not the same as long-term custodial care.
• Hospice under Part A focuses on comfort and support for end-of-life care; regular Part A/B cost-sharing can be reduced in hospice settings for covered services.

What Part B covers and the “20 percent” that matters

Part B covers outpatient and doctor services, preventive care, durable medical equipment, and certain drugs administered in a clinic or hospital outpatient setting. There’s a monthly premium and an annual deductible; after that, beneficiaries typically pay 20% coinsurance for many services if there’s no supplemental coverage. That 20% might be modest or substantial depending on your health needs, which is why many folks add Medigap or choose an Advantage plan. Enrollment timing can trigger late penalties, so I keep the Social Security enrollment pages close by—Social Security handles Part B enrollment; their primer lives on SSA.gov.

• Preventive services (wellness visits, many screenings) are often covered without cost-sharing when certain criteria are met.
• Physician and outpatient facility charges can be separate; understanding where a service is billed helps avoid surprise coinsurance.
• High-income surcharges (IRMAA) can apply to Part B (and Part D). The tables and appeals info sit on SSA’s site.

Part C is “one card” convenience with trade-offs

Medicare Advantage plans are offered by private insurers approved by Medicare. They must cover all Part A and B services, and most include Part D. Many add extras like dental, vision, and hearing benefits. The catch is that benefits are delivered under plan rules: networks, referral requirements, and prior authorization. I’ve learned to ask three pragmatic questions before choosing a plan: are my doctors in-network, are my medications on the plan’s formulary at tolerable tiers, and is the plan’s out-of-pocket maximum realistic for me? The official plan comparison tool at Medicare Plan Finder makes those checks concrete.

• Pros: Single member ID card, annual out-of-pocket cap for Part A/B services, often bundled extras.
• Cons: Network and authorization rules, potential variability year to year, and formulary differences that affect drug costs.
• Tip: Read the Evidence of Coverage (EOC) and Summary of Benefits before enrolling; they spell out copays, referrals, and rules in plain language.

Part D is where formularies and tiers do the heavy lifting

Part D plans are private plans that cover outpatient prescription drugs. Whether bundled in Advantage or standalone, each plan has a formulary (its list of covered drugs), organized in tiers that determine what you pay. Two people on the same plan can have very different costs if their meds sit on different tiers. I’ve found it helpful to keep my medication list handy and use the Plan Finder to filter by total estimated annual cost—not just the monthly premium. For a quick primer on formularies, tiers, and prior authorization, the prescription coverage pages on Medicare.gov are straightforward.

• Pharmacy choice matters: some plans use preferred pharmacies with lower copays.
• Coverage stages (deductible, initial coverage, catastrophic) can change your share during the year; the Plan Finder estimates these transitions.
• Extra Help (also called LIS) can reduce Part D premiums and copays for those who qualify; applications start at SSA.

Medigap is a backstop for Original Medicare

If you keep Original Medicare, a Medigap (Medicare Supplement) policy can help pay Part A and B deductibles and coinsurance. These are standardized lettered plans (like Plan G, Plan N) sold by private insurers. Your personal sweet spot depends on how much predictability you value versus monthly premium. Timing matters: there’s a six-month Medigap Open Enrollment Period that begins when you’re both 65 or older and enrolled in Part B, during which you generally have “guaranteed issue” rights. For details (and a truly readable guide), the federal booklet “Choosing a Medigap Policy” is well worth a look on Medicare.gov.

• Medigap works only with Original Medicare, not with Medicare Advantage.
• Benefits are standardized by letter, but prices are not; shop across insurers and check how premiums are rated (community, issue-age, attained-age).
• Some states offer additional consumer protections beyond federal rules; your State Health Insurance Assistance Program (SHIP) can explain state-specific nuances.

Enrollment windows that keep you penalty free

Medicare isn’t just what you pick but when you pick it. I wrote these on a sticky note:

• Initial Enrollment Period (IEP): a 7-month window around your 65th birthday (3 months before, the month of, and 3 months after). This is your default on-ramp for Parts A and B if you’re not actively covered by qualifying employer group insurance.
• General Enrollment Period (GEP): January 1–March 31 each year if you missed your IEP and don’t qualify for a Special Enrollment Period; coverage usually starts the month after you enroll.
• Annual Election Period (AEP): October 15–December 7 each year to join, switch, or drop Medicare Advantage and Part D plans for the following year.
• Medicare Advantage Open Enrollment Period (MA OEP): January 1–March 31 to switch MA plans or go back to Original Medicare (one change).
• Special Enrollment Periods (SEPs): triggered by events like losing employer coverage, moving out of a plan’s service area, qualifying for Extra Help, and more. These are where the rules get nuanced; I’ve leaned on the “Get help” pages at Medicare.gov and my local SHIP.

How I compare the two main pathways

Here’s the thought process I’ve used to help loved ones—no hype, just trade-offs:

• If you value wide provider choice (snowbirds, multi-state care, specialist access without referrals), Original Medicare + Medigap + Part D often fits.
• If you prefer a budget cap for Part A/B services and like one card (and you’re comfortable with networks), a strong Medicare Advantage plan can work well.
• If you take expensive brand-name meds, prioritize the Part D formulary (or the bundled MA plan’s formulary) and your pharmacy options before anything else.
• If you travel internationally, some Medigap plans offer limited foreign travel emergency benefits; many Advantage plans do not—check plan documents.
• If you have employer/union retiree coverage, call the plan’s benefits office before you enroll; their rules can change what “best” looks like.

Little habits that saved me headaches

To keep it human: I made a one-page “Medicare snapshot” for my family—current plan names, member IDs, premium amounts, pharmacies, and doctors. I calendar “shop and compare” in early October, then again each January to confirm any changes actually went through. I also save PDFs of the plan’s formulary and provider directory in case websites change mid-year. When something looks off on a bill, I log into my MyMedicare account to compare the claim with the provider’s statement. If a prior authorization is denied, I note the reason code and appeal rights; Advantage plans must outline these steps in their Evidence of Coverage. And if I hit a wall, I call my SHIP counselor—every state has free, unbiased help via the State Health Insurance Assistance Program at shiphelp.org.

• Keep a medication list with dose, frequency, and prescriber; update it before plan shopping.
• Look up preventive services you’re due for and confirm if the visit will be coded as “preventive.”
• Before a scheduled procedure, ask: will this be billed as inpatient (Part A) or outpatient/observation (Part B)? It changes your costs.

Costs change, so I treat numbers as living

Premiums and deductibles are updated yearly. I avoid memorizing amounts and instead bookmark the official cost pages and Social Security’s IRMAA page. For drug costs, the Plan Finder’s “total yearly cost” (premium + copays across coverage stages) proved more honest than any single number. If you’re facing high drug expenses, check Extra Help and manufacturer patient assistance programs; the official prescription coverage pages on Medicare.gov explain the basics, and applications for Extra Help start at SSA.gov.

• IRMAA can raise Part B and D premiums based on income; appeals are possible if your income has dropped due to life events (retirement, divorce, etc.).
• Medicaid and Medicare together (dual eligibility) can dramatically change out-of-pocket costs; your state Medicaid office or SHIP can explain options.
• Prior authorization rules, especially in Advantage plans, can change annually; re-read your EOC each fall.

Signals that tell me to slow down and double-check

There are moments when “close enough” isn’t enough:

• You’re delaying Part B due to employer coverage: confirm the employer plan counts as creditable coverage for Part B and Part D to avoid penalties. The Social Security pages at SSA.gov explain the forms and timing.
• Your meds moved tiers or dropped from the formulary: request a coverage determination or exception and ask your prescriber about therapeutic alternatives.
• You’re considering a plan for a specific specialist or hospital: verify they’re in-network and in the plan’s service area for the coming year.

A quick map of who does what

One last clarity trick: know the agency roles. The Centers for Medicare & Medicaid Services (CMS) runs Medicare policy. The Social Security Administration (SSA) enrolls most people and handles premiums and income-related adjustments. Your state SHIP provides free counseling. The federal marketplace (HealthCare.gov) isn’t where you buy Medicare plans; Medicare uses its own channels. For plan shopping and comparisons, I go straight to the official tool at Medicare Plan Finder and then confirm details with the insurer.

What I’m keeping and what I’m letting go

What I’m keeping: a calendar reminder each fall to re-check plans, a habit of verifying network and formulary details before switching, and a short list of trusted sources. What I’m letting go: the myth that “one plan is best for everyone,” and the fear of asking “obvious” questions. The more I read the official materials, the more I appreciate their practicality—dry, yes, but honest. If you use one resource this week, make it the official handbook and the plan comparison tool. And if the rules start to blur, call your SHIP counselor; that one conversation can save hours.

FAQ

1) What’s the real difference between Original Medicare and Advantage?
Answer: Original Medicare (A and B) is federal coverage accepted widely, often paired with Medigap and a standalone Part D. Advantage (C) is a private plan alternative that must cover A and B services, usually includes Part D, and uses networks and plan rules. Compare with the official tool at Medicare.gov.

2) Do I have to sign up for Part B at 65?
Answer: Many people do, but if you have qualifying employer group coverage, you might delay Part B without a penalty. The exact rules and forms live on SSA.gov; confirm before delaying.

3) How do I lower my drug costs?
Answer: Use the Plan Finder to compare total yearly costs (not just premiums), ask your prescriber about tier-preferred alternatives, check pharmacy options, and see if you qualify for Extra Help at SSA.gov.

4) Can I have Medigap and a Medicare Advantage plan at the same time?
Answer: No. Medigap only works with Original Medicare. Advantage plans have their own cost-sharing rules and do not use Medigap. The Medigap guide on Medicare.gov explains this clearly.

5) Who can help me one-on-one without trying to sell me something?
Answer: Your State Health Insurance Assistance Program (SHIP) offers free, unbiased counseling. Find your local office at shiphelp.org.

Sources & References

• Medicare & You Handbook
• Medicare Plan Finder
• SSA Medicare Basics
• Medicare Part D Overview
• State Health Insurance Assistance Program

This blog is a personal journal and for general information only. It is not a substitute for professional medical advice, diagnosis, or treatment, and it does not create a doctor–patient relationship. Always seek the advice of a licensed clinician for questions about your health. If you may be experiencing an emergency, call your local emergency number immediately (e.g., 911 [US], 119).