Quick Answer: Medigap, short for Medicare Supplement Insurance, is a private policy that works alongside Original Medicare to pay many of the deductibles, copays, and coinsurance costs Medicare leaves behind. Medigap plans are standardized into lettered plans, from A through N, and federal law requires a given letter to cover the exact same benefits no matter which company sells it. Once you pick a letter, the only real difference between carriers is price.
Once you start researching how to fill the gaps in Original Medicare, you’ll run into “Medigap” everywhere, often followed by a letter that means nothing to you yet: Plan G, Plan N, Plan F. It can feel like walking into a conversation already in progress.
Medigap is one of the more straightforward parts of Medicare once you understand the underlying structure. Below, I’ll explain what Medigap plans are, how the lettering system works, what each plan does, and how Medigap compares to Medicare Advantage. If you’re leaning toward Plan G, our deep-dive on Plan G covers it in more detail, and if you’re still deciding between Medigap and Advantage broadly, our comparison guide walks through that decision.
What Is Medigap?
Medigap, or Medicare Supplement Insurance, is a private policy that works alongside Original Medicare Parts A and B. When you visit a doctor or hospital, Medicare pays its share first, and the remaining balance is sent to your Medigap insurer, which pays per your policy’s benefits.
Medigap only works with Original Medicare, not Medicare Advantage, and covers one person per policy, so spouses need separate policies. It doesn’t include drug coverage, so most holders pair it with a standalone Part D plan. As long as you keep paying your premium, Medigap is guaranteed renewable, so the insurer can’t cancel it just because you’ve started filing claims.
Why Does Medigap Exist?
Original Medicare covers a lot, but it isn’t complete. Part B coinsurance is 20% of the Medicare-approved amount with no annual cap, so a serious hospitalization could leave you with thousands in coinsurance and no ceiling on how high that climbs. Medigap exists to close that gap and put a predictable boundary around what you’ll actually pay.
How Do Medigap Plan Letters Work?
Medigap plans are standardized by federal law into 10 lettered plans in most states: A, B, C, D, F, G, K, L, M, and N. (Massachusetts, Minnesota, and Wisconsin use their own standardized structures.) Each letter represents a fixed set of benefits defined by the federal government, identical no matter which insurance company sells it.
This is the single most important thing to understand about Medigap: a Plan G from one company covers exactly the same benefits as a Plan G from any other company selling it in your state. The only differences between carriers are premium, financial strength, and service. Choosing a letter is really a coverage decision; choosing a carrier within that letter is really a price decision.
What Do the Different Letters Actually Cover?
At a high level, Plans A through G lean toward comprehensive coverage with higher premiums and fewer surprises. Plans K, L, and M use cost-sharing: lower premiums in exchange for paying a percentage of costs yourself. Plan N sits in between, offering coverage similar to Plan G but with small copays and no Part B excess charge coverage, for a lower premium.
Plans C and F, once the most comprehensive because they covered the Part B deductible, closed to anyone newly eligible on or after January 1, 2020. If you’re new to Medicare, Plan G is now the most comprehensive letter available, which is why it’s the most commonly purchased. For what Plan G covers and costs, see our Plan G guide.
What Do Medigap Plans Actually Pay For?
Across the standardized letters, Medigap benefits are built from the same building blocks: Part A hospital coinsurance and 365 extra hospital days, Part B coinsurance, the first three pints of blood, Part A hospice coinsurance, skilled nursing facility coinsurance, the Part A deductible ($1,736 in 2026), Part B excess charges, and a foreign travel emergency benefit. Which of these a letter includes, and to what percentage, is what differentiates the plans.
None of the standardized plans include drug coverage. You’ll need a separate, standalone Part D plan, which has a $2,100 annual out-of-pocket cap in 2026.
A Trade-Off Worth Understanding
Comprehensive letters like Plan G cost more monthly but leave very little unpredictable cost when you need care. Leaner letters like Plan N or the cost-sharing plans cost less monthly but leave you carrying more of the bill yourself. There’s no universally “best” letter, only the one that matches how much premium versus out-of-pocket exposure you’re comfortable carrying.
When Can You Enroll in a Medigap Plan Without Medical Underwriting?
Timing matters enormously with Medigap. Your Medigap Open Enrollment Period is a one-time, six-month window starting the first day of the month you’re both 65 or older and enrolled in Part B. During this window, insurers must sell you any Medigap plan they offer at their standard rate, regardless of health history, with no medical underwriting.
Outside that window, insurers can generally use medical underwriting, meaning they can charge more or deny coverage based on your health. Certain qualifying events, like your Medicare Advantage plan leaving your area, can open a separate guaranteed issue window, but these are narrower and situation-specific.
A Real Scenario Worth Considering
Say you’re turning 65 in Tulsa this fall, generally healthy now, but with a family history that could complicate underwriting later. Enrolling during your six-month window locks in guaranteed acceptance regardless of what happens with your health later. Waiting can mean paying more, or being denied coverage, if your health changes.
Medigap vs. Medicare Advantage: How Are They Different?
Medigap and Medicare Advantage fill the same gaps in different ways. Medigap works alongside Original Medicare and lets you see any doctor nationwide who accepts Medicare, with no networks or referrals, but costs more monthly and requires a separate Part D plan.
Medicare Advantage replaces Original Medicare entirely with a private plan that usually bundles in drug coverage and extras like dental and vision, often for a low or $0 premium, but typically restricts you to a network. You cannot have both at once. For a full side-by-side breakdown, see our Medicare Advantage vs. Medicare Supplement comparison.
How Do You Choose the Right Medigap Plan?
Start by deciding whether Medigap fits you at all, meaning you value provider flexibility and cost predictability enough to pay a higher premium. From there, the letter you choose comes down to how much monthly cost you’re willing to carry versus how much out-of-pocket risk you’re comfortable with.
Because every carrier selling the same letter offers identical coverage, the most valuable thing you can do is compare actual rates across carriers rather than assuming price differences reflect coverage differences. This is exactly the kind of comparison I walk clients in the Tulsa area through regularly. If you’d like help narrowing down your options, you can schedule a free consultation and we’ll compare Medigap against Medicare Advantage, and Plan G against the other letters, for your situation.
Frequently Asked Questions
What are Medigap plans?
Medigap, or Medicare Supplement Insurance, is a private policy that works alongside Original Medicare to pay many of the costs, like deductibles and coinsurance, that Medicare leaves behind.
How do Medigap plan letters work?
Medigap is standardized by federal law into lettered plans (A through N in most states), and each letter is a fixed set of benefits that’s identical no matter which company sells it. Only the premium and carrier differ.
What is the difference between Medigap and Medicare Advantage?
Medigap works alongside Original Medicare with nationwide provider access and no networks, but costs more and excludes drug coverage. Medicare Advantage replaces Original Medicare with a network-based plan, often at a lower premium with bundled extras.
When can I enroll in Medigap without medical underwriting?
During your one-time, six-month Medigap Open Enrollment Period, which starts the first day of the month you’re 65 or older and enrolled in Part B. Outside that window, insurers can generally use medical underwriting.
Which Medigap plan is most popular?
Plan G is currently the most commonly purchased Medigap plan for new enrollees, since Plans C and F closed to anyone newly eligible for Medicare on or after January 1, 2020.
The Bottom Line
Medigap comes down to one core idea: a private policy that pays many of the costs Original Medicare leaves behind, standardized so that choosing a letter is a coverage decision and choosing a carrier is a price decision. That structure is the foundation for every more specific Medigap decision you’ll make.
If you’re ready to narrow down which letter and carrier fit your situation, call 918-815-6382 or schedule a free consultation and we’ll walk through your options. You can also read our Plan G deep-dive, our Advantage vs. Supplement comparison.