The four parts, plainly
A, B, C, D — what each covers.
Part A is hospital insurance. Part B is medical/doctor visits. Part C (Medicare Advantage) bundles A+B through a private plan, often with extras. Part D is prescription drug coverage. Original Medicare is A+B; you add D and/or a Medigap supplement to fill gaps.
Watch for: missing an enrollment window. Late Part B or D enrollment can mean a permanent penalty added to your premium. The windows matter.
Medigap vs. Medicare Advantage
The biggest decision you'll make.
Medigap (supplement) works alongside Original Medicare — you can see any provider who takes Medicare, and it pays some of what Medicare doesn't. Medicare Advantage replaces Original Medicare with a private plan that usually has a network and often bundles drug coverage.
Watch for: networks. Advantage plans can restrict which doctors you see. Medigap gives you more freedom but costs more per month. Weigh access against premium.
Your enrollment windows
The dates that actually matter.
Your Initial Enrollment Period starts 3 months before you turn 65 and runs 3 months after. Every fall, Open Enrollment (AEP, Oct 15 – Dec 7) lets you switch Advantage or Part D plans. Medigap Open Enrollment is a 6-month window that starts the month you're 65+ and enrolled in Part B — during it, insurers can't deny you or charge more for health.
Watch for: the Medigap 6-month window. Miss it and, in most states, insurers can underwrite you and turn you down. It's the most valuable window you have.