You may have heard of a Medigap policy. But what is it, exactly?

Also known as Medicare supplement insurance, a Medigap policy is a type of private insurance policy. It’s designed to help fill the "gaps" in coverage left by original Medicare (Part A and Part B). Medigap policies are sold by private carriers. They’re meant to work alongside Medicare to provide additional benefits and help cover some of the costs Medicare does not pay for.

"A Medigap policy can give Medicare beneficiaries greater peace of mind by covering out-of-pocket costs like deductibles and coinsurance," said Ryan Ramsey, NCOA Associate Director of Health Coverage and Benefits. "It helps create financial predictability, especially for those with frequent medical needs or on a fixed income."

When can I buy a Medigap plan?

The best time to buy a Medigap policy is when you’re 65 or older and you first sign up for Medicare Part A and Part B. Typically, you need this coverage to buy a Medigap policy. This period is your Medigap Open Enrollment Period. You have six months after your Part B coverage starts to buy any Medigap policy sold in your state (even if you have pre-existing health problems).

In most cases, you cannot change your Medigap coverage outside your Medigap Open Enrollment Period. In every state except Connecticut, Maine, Massachusetts, and New York, if you apply for a Medigap policy after your first six months of Medicare eligibility, the insurance company can deny coverage (or charge you more due to a pre-existing condition). "A Medigap insurer can generally turn you down for whatever reason they see fit," Ramsey told Fortune Well.

Medicare.gov has an online tool to help you find out if you have a legal right to switch or drop Medigap policies.

If you’re under age 65, you might not be able to buy a Medigap policy at all, or it may be more expensive. This is true even if you have Medicare and buy Medigap in the first six months after your Medicare coverage starts.

How can I choose a Medigap plan that’s right for me?

With so many options, choosing the right Medigap plan can feel overwhelming. Start by gathering some basic information. You can narrow down your choices simply by asking the right questions. Below are some questions to use when you’re speaking to insurance representatives about Medigap:

1. Am I enrolling while I am in my Medigap Open Enrollment Period? If not, do I have a guaranteed issue right?
2. What is the Medigap policy’s monthly premium?
3. Is my premium based on my health status, sex, smoking status, marital status, or anything else?
4. Are the premiums:
   
   No-age-rated (community-rated), meaning everyone in my area pays the same premium regardless of their age?
   
   Issue-age-rated, meaning the premium is based on how old I am when I buy the policy?
   
   Attained-age-rated, meaning the premium increases based on my age?
5. Will the company refuse to sell me a Medigap policy based on my health status?
6. Does the policy impose a pre-existing condition waiting period?
7. How long is the waiting period before my coverage begins?
8. Do I have prior creditable coverage to reduce my waiting period?

If you are under 65, make sure the company you are considering sells to people under 65. Also, keep track of who you spoke with, when you spoke with them, and the outcome of the call.

Ready to browse Medigap plans? You can explore different Medigap policy options using Medicare.gov's Medigap Policy Finder.