Medicare's Alphabet, Decoded: Parts A, B, C, and D — and Where Medigap Fits

Part A: hospital coverage. Part A covers inpatient hospital stays, care in a skilled nursing facility following a qualifying hospital stay, some home health services, and hospice care. For most people, Part A is premium-free at 65 because they or their spouse paid Medicare taxes for a sufficient number of working years. Free doesn't mean cost-free at the point of use — Part A has its own deductibles and cost-sharing for hospital stays — but the monthly premium burden is usually zero. People who don't have the required work history can still get Part A but pay a monthly premium for it. Part A alone is not comprehensive coverage.

Part B: medical coverage. Part B covers outpatient care: visits to doctors, specialists, and other providers; preventive services; lab work; durable medical equipment; and outpatient procedures. Unlike Part A, Part B has a monthly premium that most enrollees pay. That premium is income-adjusted — higher earners pay more through a surcharge system called IRMAA, which is assessed based on your tax return from two years prior. Part B also has an annual deductible and then covers a percentage of approved costs, leaving the remainder as your responsibility. Together, Parts A and B form what's called Original Medicare, and that cost-sharing is why many people layer additional coverage on top.

Medigap: filling the gaps in Original Medicare. Medigap (also called Medicare Supplement) is private insurance sold by insurance companies to cover some or most of the cost-sharing that Original Medicare leaves behind — deductibles, copays, and coinsurance. Plans are standardized by letter (Plan G, Plan N, and so on), which means a Plan G from one insurer covers the same things as a Plan G from any other insurer; what varies is the monthly premium each company charges for that letter. The practical appeal: with a robust Medigap plan, your out-of-pocket exposure on most medical services becomes predictable, sometimes close to zero beyond the monthly premium. The tradeoff is a higher premium than you'd pay under alternatives, and Medigap doesn't cover prescriptions — that requires a separate Part D plan.

Part D: prescription drug coverage. Part D covers outpatient prescription drugs and is sold by private insurers as standalone plans that pair with Original Medicare, or bundled into Medicare Advantage. Each Part D plan has its own formulary — the list of drugs it covers and at what tier of cost-sharing — so the right plan depends on your specific medications. Formularies can change year to year, which is why reviewing your Part D coverage annually during the fall open enrollment window is worth doing rather than ignoring. Part D has a late enrollment penalty if you go without creditable drug coverage for an extended period after becoming eligible, and that penalty is permanent, added to your premium for as long as you have Part D.

Part C (Medicare Advantage): the private alternative. Medicare Advantage plans are offered by private insurers that contract with Medicare. Enrolling in Part C means getting your Part A, Part B, and usually Part D coverage through the private plan rather than directly through the government. Advantage plans often include extra benefits that Original Medicare doesn't cover — dental, vision, hearing, and fitness programs appear frequently. The structure usually involves a network of providers and different cost-sharing rules than Original Medicare. For some people, a well-matched Advantage plan at a lower premium is clearly the better fit. For others — particularly those with complex conditions, preferred specialists, or frequent travel — Original Medicare's broader provider access matters more. This is a genuine tradeoff with no universal right answer.

Original Medicare plus Medigap versus Medicare Advantage. The core tradeoff between these two paths comes down to predictability versus flexibility versus cost. Original Medicare with a strong Medigap plan tends to offer broad provider access — nearly any provider that accepts Medicare — and highly predictable out-of-pocket costs, but typically at a higher combined premium. Medicare Advantage often comes with lower or zero additional premium beyond Part B and includes extra benefits, but usually requires using a network, needs a referral system in many plans, and exposes you to more variable out-of-pocket costs depending on utilization. The comparison isn't about which is objectively better; it's about which structure matches your healthcare usage, geographic situation, and financial preferences.

Enrollment windows and penalties — the mechanics that matter. Your Initial Enrollment Period around age 65 is a seven-month window — three months before your birthday month, your birthday month, and three months after. Enrolling in Part B late without creditable employer coverage results in a permanent premium surcharge, assessed for each year you were without coverage, added to your Part B premium indefinitely. The same principle applies to Part D. Medigap has its own enrollment mechanics: there's a guaranteed-issue period around initial Medicare enrollment when insurers must sell you any plan regardless of health history; after that window closes, medical underwriting can apply in most states. Medicare.gov publishes the official enrollment calendars, plan comparison tools, and the annual Medicare & You handbook, which is the most authoritative plain-language reference available.