Here is the single most useful fact about Medicare Supplement insurance, the coverage everyone calls Medigap: the plans are standardized by letter. A Plan G sold by one insurer covers exactly what a Plan G sold by any other insurer covers, in almost every state. The benefits are fixed by law; the only things that vary are the premium and the company’s service. Which means shopping for Medigap is really two separate questions: which letter, then whose price.

That’s genuinely good news for buyers, and it’s buried under some of the most confusing marketing in all of insurance. Here’s what the letters mean, which ones people actually compare in 2026, and the timing rule that matters more than any of it.

What Medigap is, and one thing it isn’t

Original Medicare pays generously but not completely: Part A and Part B leave you with deductibles and, most significantly, a 20 percent coinsurance on most outpatient care, with no annual cap. Medigap policies are private insurance that fills those gaps, paying some or all of what Original Medicare leaves behind. Medicare.gov’s plan-comparison chart shows every letter’s benefits side by side.

The thing it isn’t: Medicare Advantage. Advantage plans replace how you get Original Medicare; Medigap supplements Original Medicare. You cannot use a Medigap policy to pay Advantage-plan costs, and it’s illegal for anyone to sell you one if you’re in an Advantage plan. You pick one road or the other.

Medigap also doesn’t cover prescription drugs. That’s Part D, a separate policy with its own premium.

Reading the letters

Ten standardized plans exist: A, B, C, D, F, G, K, L, M and N. (Massachusetts, Minnesota and Wisconsin standardize their plans differently.) You don’t need to memorize the grid; you need the shape of it:

Plan A is the floor: it covers the Part A hospital coinsurance and some other basics, and every insurer that sells Medigap must offer it. Plans B, D and M add pieces, mainly around the Part A deductible.

Plans F and G are the comprehensive ones. Plan F historically covered essentially everything, including the Part B deductible. But under a federal rule, plans that cover the Part B deductible can’t be sold to anyone who became eligible for Medicare on or after January 1, 2020. So F (and C) are closed to newcomers; people who already had them keep them.

Plan G is the most complete plan a new retiree can buy: it covers everything F does except the Part B deductible, which is $283 in 2026 per the Centers for Medicare & Medicaid Services. Pay that once a year yourself and G behaves like F did.

Plan N trades a lower premium for small cost-sharing: copays of up to $20 for office visits and up to $50 for emergency-room trips, and it doesn’t cover Part B “excess charges,” the up-to-15-percent surcharge a minority of doctors who don’t accept Medicare’s standard payment amounts are allowed to bill.

Plans K and L pay a percentage of most gaps (50 and 75 percent respectively) rather than all of them, in exchange for lower premiums, and they’re the only letters with an annual out-of-pocket limit, after which they pay 100 percent. The limits adjust each year; check the current figures on Medicare.gov’s chart.

The high-deductible versions

Plans F and G also come in high-deductible flavors: identical coverage that only kicks in after you’ve paid a deductible yourself, $2,950 for 2026, per CMS. Premiums are dramatically lower. For someone with the savings to absorb a bad year, high-deductible G is essentially catastrophic coverage: you self-insure the routine gaps and cap your worst case around the deductible plus premiums. It’s the sleeper option of the lineup and worth pricing before you dismiss it.

The window that matters more than the letter

Now the part to put on the calendar. Your Medigap Open Enrollment Period runs for six months, starting the first month you are 65 or older and enrolled in Part B. During that window, insurers must sell you any plan they offer at the standard price, regardless of your health. No medical questions that matter, no denials, no surcharges for your diabetes or your heart history.

After the window closes, in most states, the protection flips: insurers can generally use medical underwriting to charge you more or turn you down entirely, outside of certain guaranteed-issue situations (like your insurer leaving the market, or losing employer coverage). A healthy 70-year-old can usually still switch plans; one with a serious diagnosis often can’t. That asymmetry is why the standard advice is to buy the most coverage you expect to want during your open enrollment, rather than planning to trade up later. Later may not be for sale.

How to actually shop

Pick the letter first, using the comparison chart; for most new buyers in 2026 the realistic short list is G, N and high-deductible G. Then compare premiums for that one letter across insurers using Medicare.gov’s Medigap search tool, which shows the companies selling in your ZIP code. Since benefits are identical within a letter, a higher premium for the same letter buys you nothing but the name on the card. Do ask how the insurer prices by age, since some policies are designed to rise as you get older while others spread the cost level over time; your state insurance department can explain which methods are sold locally.

One honest caveat: premiums vary by state, age and insurer, and no article can tell you your number. The tool can, in about ten minutes. Letters first, then price. That order turns one of retirement’s most confusing purchases into one of its most straightforward.

This article was produced with AI assistance and reviewed by a human editor. Figures are linked to their primary sources; where a claim could not be verified from the public record, we say so.