Medicare Explained: Parts A, B, C, and D

Medicare is one of the most consequential insurance decisions most Americans will make, and it is also one that arrives with a deadline. The initial enrollment period for Medicare is a specific seven-month window centered on your 65th birthday, and missing it can result in permanent premium penalties that increase your monthly cost for the rest of your life.

Despite this urgency, many people approaching 65 have only a vague understanding of Medicare's structure: four distinct parts with different coverage, different costs, and different enrollment rules that interact in ways that are not intuitively obvious. The decision between Original Medicare and Medicare Advantage, the role of Medigap supplemental coverage, and the mechanics of Part D prescription drug coverage all require specific knowledge that most people are acquiring for the first time under time pressure.

This guide explains Medicare's four parts clearly, the key decisions you will need to make around initial enrollment, the differences between Original Medicare and Medicare Advantage, and the most common mistakes to avoid.

The Four Parts of Medicare: What Each Covers

Part A covers inpatient hospital care, skilled nursing facility care following a qualifying hospital stay, hospice care, and some home health services. Most people receive Part A premium-free if they or their spouse have worked and paid Medicare taxes for at least 40 quarters. The Part A deductible, which applies per benefit period rather than annually, is substantial and may be covered by Medigap or Medicare Advantage.

Part B covers outpatient medical services including physician visits, preventive care, medical equipment, outpatient surgery, and most medical services not provided in a hospital setting. Part B has a monthly premium, which is income-adjusted for higher earners through the IRMAA surcharge, and an annual deductible. After the deductible, Medicare pays 80 percent of approved costs and you pay 20 percent with no cap unless you have supplemental coverage.

Part C, Medicare Advantage, is an alternative to Original Medicare Parts A and B delivered through private insurance companies approved by Medicare. These plans must cover everything Medicare covers and typically include additional benefits like dental, vision, and hearing. They use network-based cost-sharing structures similar to commercial plans and often include Part D drug coverage. Part D is standalone prescription drug coverage available to people on Original Medicare.

Original Medicare vs Medicare Advantage: The Core Choice

Original Medicare, meaning Parts A and B together, provides coverage that works with virtually any provider nationwide who accepts Medicare, without network restrictions. This nationwide acceptance makes it particularly valuable for people who travel frequently, who have established relationships with specific providers, or who live part-year in different locations.

The significant limitation of Original Medicare is the absence of an out-of-pocket maximum. The 20 percent coinsurance under Part B applies without an annual cap, meaning a serious illness or significant hospitalization can produce substantial financial exposure. Medigap supplemental insurance, also called Medicare Supplement insurance, addresses this gap by covering some or all of the cost-sharing that Original Medicare leaves the enrollee responsible for.

Medicare Advantage plans provide an all-in-one alternative that includes the Medicare benefits plus often dental, vision, hearing, and sometimes fitness benefits, typically with a lower or zero premium and an annual out-of-pocket maximum. The trade-off is network restrictions, prior authorization requirements, and plan designs that change annually. Medicare Advantage plans vary significantly by market, and the quality and breadth of available plans depends on your geographic area.

Enrollment Periods and Penalties

The initial enrollment period for Medicare is a seven-month window: the three months before your 65th birthday month, your birthday month itself, and the three months after. Enrolling during the first three months of this window ensures coverage begins on your birthday month. Waiting until the second half of the window delays coverage start.

Missing your initial enrollment period results in late enrollment penalties that are permanent. Part B late enrollment adds 10 percent to the standard premium for each 12-month period you were eligible but not enrolled without creditable coverage. Part D late enrollment adds 1 percent of the national base beneficiary premium for each month of late enrollment. These penalties never go away.

Qualifying for a special enrollment period, such as when you or your spouse have employer group health coverage through active employment, can extend the enrollment window. People who remain on employer coverage past 65 because they or their spouse are still working can typically delay Medicare enrollment without penalty. Retiree coverage from a former employer is generally not active employment coverage for this purpose.

Medigap: Filling Original Medicare's Gaps

Medigap plans, also called Medicare Supplement plans, are private insurance policies that work alongside Original Medicare to cover cost-sharing that Medicare leaves to the enrollee. Plans are standardized by letter designation across most states, meaning Plan G from one insurer covers the same benefits as Plan G from another, though premiums differ.

The best time to purchase Medigap is during your Medigap open enrollment period, which is the six months beginning when you are both 65 and enrolled in Part B. During this window, insurers cannot deny you coverage or charge you more based on health conditions. After this window closes, medical underwriting applies in most states, meaning you may be declined or charged higher premiums based on health history.

Plan G is currently one of the most popular Medigap options, covering most cost-sharing including Part B excess charges, with the exception of the Part B deductible. Plan N provides somewhat lower premiums in exchange for copayments at office visits and emergency rooms. Plans with lower premiums and higher cost-sharing can work for people who use relatively little healthcare; richer plans provide more predictable costs for people with significant needs.

Final Thoughts

Medicare is a foundational element of financial security for Americans 65 and older, and understanding its structure before you need to make enrollment decisions is one of the most valuable healthcare planning steps you can take as you approach that milestone.

The core decisions, Original Medicare versus Medicare Advantage, whether to add Medigap, and which Part D plan fits your prescriptions, require specific knowledge about your own health situation, provider preferences, and financial priorities. Making these decisions with adequate information during the right enrollment windows protects you from lifetime premium penalties and ensures you have coverage that actually fits your needs.

Start learning the system at least a year before you turn 65. The decisions you make at enrollment affect the rest of your healthcare life.