Parts A, B, C, D, Medigap, IRMAA: Medicare vocabulary decoded

Medicare is the federal health program for people 65 and older (and some younger people with disabilities), and it's organized as an alphabet — Parts A, B, C, and D — plus supplemental Medigap plans and an income surcharge called IRMAA. The structure is confusing by design of history, not malice, and the enrollment deadlines carry lifelong penalties for missing them. Here's the map. Rules and figures change; verify current details at medicare.gov and consider professional guidance.

The four parts

• Part A (hospital insurance) — inpatient hospital stays, skilled nursing, hospice. Most people pay no premium because they paid Medicare taxes while working.
• Part B (medical insurance) — doctor visits, outpatient care, preventive services. It has a monthly premium and an annual deductible.
• Part C (Medicare Advantage) — a private-plan alternative that bundles A, B, and usually D, often with extra benefits and a network, in place of Original Medicare.
• Part D (prescription drug coverage) — private plans covering medications, with their own premiums and formularies.

The supplement and gap words

• Medigap (Medicare Supplement) — private policies that pay Original Medicare's out-of-pocket costs (deductibles, coinsurance). Sold in standardized lettered plans; works only with Original Medicare, not Advantage.
• Original Medicare has no out-of-pocket maximum — which is why many pair it with Medigap; Advantage plans do have an annual cap.
• Formulary — a Part D or Advantage plan's list of covered drugs, tiered by cost; the same medication can cost very different amounts across plans.
• Coverage gap ('donut hole') — a historical Part D phase of higher drug costs; recent reforms have added an annual out-of-pocket cap on Part D spending.

The income surcharge

IRMAA (Income-Related Monthly Adjustment Amount) is a surcharge added to Part B and Part D premiums for higher-income beneficiaries, based on your tax return from two years prior. It creates income 'cliffs' — cross a threshold by even a little and your premiums jump. This is why retirees manage taxable income (through Roth conversions timed earlier, QCDs, and capital-gains planning) to avoid tripping IRMAA brackets, and why a one-time income spike two years before can raise Medicare costs unexpectedly.

Approaching Medicare

1. Mark your Initial Enrollment Period around age 65 and enroll on time to avoid lifelong penalties.
2. Decide early between Original Medicare (+ Medigap + Part D) and Medicare Advantage, knowing switching back can require Medigap underwriting.
3. Use the Medigap open enrollment window if you want a supplement — it's the one time you're guaranteed coverage regardless of health.
4. Check each Part D or Advantage plan's formulary against your actual medications, not just the premium.
5. Manage income two years ahead to avoid IRMAA cliffs; a professional can help time conversions and withdrawals.

The bottom line

Medicare's alphabet is really two big choices wrapped in vocabulary: which path you take (Original Medicare with a supplement, or all-in-one Advantage) and how you protect against its gaps and penalties. Enroll on time to avoid permanent surcharges, use the Medigap window while your health can't be held against you, match drug plans to your actual prescriptions, and watch the two-year-lookback IRMAA thresholds. Learn the parts before 65, and a system designed to intimidate becomes a set of manageable, if unglamorous, decisions.