Your Original Medicare insurance covers a wide variety of health services, from flu shots to hospital stays to hospice care. But it doesn’t cover everything, and it doesn’t cover all your out-of-pocket costs.

Many services covered by Original Medicare require co-payments, coinsurance, and deductibles. You can purchase extra insurance to cover these “gaps” in Medicare. Such insurance is called Medicare Supplement Insurance, or Medigap. Some Medigap policies also provide coverage that Original Medicare doesn’t, like emergency care in a foreign country.

You have to pay for Medigap yourself, and it’s sold through private insurance companies. You can buy it only if you have Original Medicare, not Medicare Advantage, which is managed care provided by private insurers.

Every Medigap policy has to follow federal and state laws designed to protect you. Insurance companies can sell you only a “standardized” Medigap policy identified in most states by the letters A through N. Each standardized policy must offer the same basic benefits, no matter which company sells it.

So beware when you’re shopping for a Medigap policy: Cost is usually the only difference between Medigap policies with the same letter sold by different companies.

And there can be big differences in how much various insurers charge for the same coverage.

Here are some of the costs that Medigap policies often cover:

* Medicare Part A (hospital) coinsurance and hospital costs for up to 365 days after Medicare benefits run out;

* Medicare Part B (medical) coinsurance or co-pays;

* Part A hospice care coinsurance or co-pays;

* Skilled nursing facility coinsurance;

* Part A and Part B deductibles.

Medigap policies generally don’t cover long-term care (like care in a nursing home), vision or dental, hearing aids, eyeglasses, and private‑duty nursing.

The best time to buy a Medigap policy is during your Medigap open enrollment period. This period lasts for six months and begins on the first day of the month in which you’re 65 or older and enrolled in Medicare Part B.

Why is this important? Because during open enrollment, an insurance company can’t refuse to sell you any Medigap policy it offers due to any health problems you may have. Nor can you be charged more based on your health status.

In some cases, however, an insurer can refuse to cover your out-of-pocket costs for pre-existing health conditions for up to six months. After six months, the Medigap policy will cover the pre-existing condition.

And remember, for Medicare‑covered services, Original Medicare will still cover the condition even if the Medigap policy won’t cover your out‑of‑pocket costs. But you’re responsible for the coinsurance or copayments.

A few other points to keep in mind:

* You must have Medicare Part A and Part B to buy a Medigap policy.

* Plans E, H, I, and J are no longer for sale, but you can keep these plans if you already have one.

* A Medigap policy only covers one person. If you and your spouse both want Medigap coverage, you must each buy a separate policy.

* Any standardized Medigap policy is guaranteed renewable, even if you have health problems. This means the insurance company can’t cancel your policy as long as you pay the premium.

Although some Medigap policies sold in the past cover prescription drugs, Medigap policies sold after January 1, 2006, aren’t allowed to include prescription drug coverage. (If you want such coverage, you can join a Medicare Part D prescription drug plan, offered by private companies approved by Medicare.)

David Sayen is Medicare’s regional administrator for Arizona, California, Hawaii, Nevada, and the Pacific Territories. You can always get answers to your Medicare questions by calling 1-800-MEDICARE (1-800-633-4227).