Original Medicare pays 80 percent of the approved cost for most Part B services, and leaves the remaining 20 percent coinsurance to the patient, with no built-in yearly cap on how high that bill can climb. That gap is not a design flaw; it is the reason a second layer of coverage exists at all. Medigap, formally Medicare Supplement Insurance, is the federally standardized product built specifically to absorb that 20 percent and the other cost-sharing Original Medicare leaves behind. The part most people miss is timing: Medigap has one guaranteed window to buy in without a health screening, and it is measured in months, not years.
What a Medigap Policy Is Built to Pay
A Medigap policy is not a stand-alone health plan; it only works alongside Original Medicare, paying toward costs Original Medicare approves but doesn’t cover in full. Ten standardized plan letters are sold today — A, B, C, D, F, G, K, L, M and N — and each packages that help differently, from copayments to hospital coinsurance. Plans C and F are closed to anyone who turned 65 on or after January 1, 2020, so most first-time buyers now choose among the eight plan letters still open to new enrollees.
Medicare.gov describes Medigap’s job in plain terms: it helps cover the copayments, coinsurance and deductibles that come with Original Medicare-approved care, and it pays second, only after Medicare has already paid its share of a claim. Some policies go further and add a benefit Original Medicare skips entirely — emergency medical care during foreign travel, covered up to plan limits on six of the ten standardized letters. That extra layer is one reason a Medigap premium runs higher than doing nothing at all, but it is priced against an uncapped 20 percent bill, not a capped one.
What a Medigap policy will not do matters just as much. Medicare.gov is explicit that these policies generally don’t cover long-term custodial care such as an extended nursing-home stay, along with routine vision, dental, hearing aids or private-duty nursing — costs many buyers wrongly assume “supplement” insurance absorbs. A Medigap policy also can’t be paired with a Medicare Advantage plan; the moment someone enrolls in Medicare Advantage, using both is against federal rules, which forces a binary choice between the two paths at enrollment.
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The Uncapped 20 Percent Behind the Bill
The gap Medigap is built to close is specific: after a person meets the annual Part B deductible — $283 in 2026 — Original Medicare pays 80 percent of the Medicare-approved amount for most outpatient care, doctor visits and durable medical equipment, leaving the remaining 20 percent coinsurance to the patient. Unlike Medicare Advantage plans, which are required to cap yearly out-of-pocket spending, Original Medicare alone carries no such ceiling on that 20 percent. A single hospitalization, a round of chemotherapy or a series of specialist visits can turn that uncapped percentage into a five-figure bill with nothing standing between the patient and the balance.
Medigap plans absorb that 20 percent at different rates, and the difference is the core decision buyers face. Plans A, B, D, G, M and N pay the Part B coinsurance in full, though Plan N carries its own small copayments — up to $20 for some office visits and $50 for an emergency-room visit that doesn’t lead to admission. Plans K and L instead split the coinsurance, paying 50 percent and 75 percent, but both carry a yearly out-of-pocket cap — $8,000 for Plan K and $4,000 for Plan L in 2026 — after which the plan pays 100 percent for the rest of the calendar year.
That structure inverts the tradeoff Medicare Advantage offers. Medicare Advantage plans build in a yearly out-of-pocket maximum by law and often cost less in premium, but they run through a provider network and require prior authorization for many services. A Medigap policy generally costs more in premium every month, but it pays predictably against Original Medicare’s own open network, with no referrals and no network map to check before a specialist visit — a tradeoff between premium size and cost certainty that plays out differently depending on how much care a person actually uses.
A Six-Month Window Decides the Price
Whether a Medigap policy is affordable at all often comes down to timing more than health. Federal law creates a one-time Medigap Open Enrollment Period: a six-month window that starts the first day of the month a person is both 65 or older and enrolled in Part B. During that window, an insurance company cannot deny an application, charge a higher premium or impose a waiting period because of a pre-existing health condition — coverage is guaranteed regardless of medical history.
Once that six-month window closes, the federal guarantee closes with it in most states, and an insurer can medically underwrite a new application, charge more for existing conditions or turn a buyer down outright. A narrower set of guaranteed-issue rights still applies afterward for specific situations — losing employer coverage, a Medicare Advantage plan leaving the market or exiting a service area, or returning to Original Medicare within a trial period — but each requires applying within a tight federal deadline, typically starting 60 days before coverage ends and closing no later than 63 days after it ends. Miss both windows without qualifying for an exception, and the purchase depends entirely on state law and an insurer’s own underwriting standards.
The practical stakes make the six-month window arguably the single highest-leverage decision in the entire Medigap purchase — more consequential than which lettered plan a buyer eventually picks. Someone who assumes Medigap can be shopped for anytime, the way an auto or homeowners policy can, risks discovering the coverage exists on paper but is priced out of reach or simply unavailable once health problems have already surfaced. The 20 percent gap in Original Medicare does not go away with age or income; the chance to close it without a health question attached does.
This article was researched and drafted with the assistance of artificial intelligence.
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