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

If you're approaching 65 or helping a loved one navigate their healthcare options, you've probably heard people refer to Medicare "parts" and wondered what they all mean. You're not alone. Medicare has a structure that can feel confusing at first, but once you understand the basics, the pieces fit together in a way that makes sense.

This guide breaks down each part of Medicare in plain language so you know exactly what's covered, what isn't, and how the parts work together.

The Four Parts of Medicare

Medicare is organized into four main parts, each covering a different category of healthcare. Think of them as building blocks that combine to form your total coverage.

Original Medicare vs. Medicare Advantage

Once you understand the four parts, the next important concept is that you have a fundamental choice to make about how you receive your Medicare benefits. There are two main paths:

Neither option is universally better. The right choice depends on your doctors, your medications, how you use healthcare, and what's available in your area. That's where working with an independent advisor who can compare plans across multiple carriers can make a real difference.

What About Medigap (Medicare Supplement)?

If you choose Original Medicare, you'll notice that Part A and Part B don't cover everything. You'll still have deductibles, copays, and coinsurance. That's where Medigap (also called Medicare Supplement insurance) comes in.

Medigap plans are sold by private insurance companies and help pay for some of the out-of-pocket costs that Original Medicare doesn't cover. There are several standardized plan types (labeled with letters like Plan G, Plan N, etc.), and the coverage is the same regardless of which company sells the plan. The difference between companies is typically the premium price and customer service.

When Can You Enroll?

Medicare has specific enrollment windows that determine when you can sign up or make changes. The most important ones to know:

Initial Enrollment Period (IEP)

A 7-month window around your 65th birthday: the 3 months before your birthday month, your birthday month, and the 3 months after. This is when you first become eligible and can enroll without restrictions.

Annual Enrollment Period (AEP)

October 15 through December 7 each year. During AEP, you can switch between Original Medicare and Medicare Advantage, change your Part D plan, or make other coverage adjustments. Changes take effect January 1.

Medicare Supplement Open Enrollment

A one-time, 6-month window that starts the month you turn 65 and are enrolled in Part B. During this period, insurance companies cannot deny you Medigap coverage or charge you more based on health conditions. After this window closes, you may face medical underwriting if you apply later.

How the Parts Fit Together

Here's a simple way to think about how everything connects:

1. Everyone starts with Part A and Part B (Original Medicare). These are the foundation.
2. You then choose a path: stay with Original Medicare and add a Medigap plan + a Part D drug plan, or switch to a Medicare Advantage plan (Part C) that bundles everything together.
3. Review your coverage annually. Your health, medications, and the plans available in your area change from year to year. Checking your options each fall during AEP can help you avoid paying more than you need to.

What's the Best Approach?

There's no single "best" Medicare plan. The right combination depends on your specific situation: your health, your preferred doctors, your prescription medications, your budget, and your tolerance for navigating provider networks.

What does make a difference is having someone who can look at your full picture, compare options across carriers, and explain the trade-offs in plain language so you can make an informed decision with confidence.