Medigap or Medicare Supplemental insurance is a private insurance policy that can be purchased to fill the ‘Gaps’ in Original or Traditional Medicare. You must have both Part A and Part B of Original Medicare in order to buy this type of policy. This type of coverage gives you the freedom to see any doctor or visit any hospital that accepts Medicare without a referral. You can also predict your medical costs more accurately because, most times, you’ll have no copays for your covered Medicare benefits.
If you have Original Medicare and you buy a Medigap policy, Medicare will pay its share of the Medicare-approved amount for covered health care costs first. Then your Medigap policy pays its share of covered benefits. Every Medigap policy must follow federal and state laws designed to protect you, and it must be clearly identified as Medicare supplemental insurance. Insurance companies that sell Medigap can only sell you “standardized” Medigap policies, identified in most states by letters. I.e. ‘G’ or ‘N’ etc…
All Medigap plans offer the same basic benefits, but some offer additional benefits. Plans identified by the same letter cover the same benefits regardless of which company sells them.
You have a one-time, six-month Medigap open enrollment period that starts the first month you’re enrolled in Part B. This period gives you a guaranteed right to buy any Medigap policy sold in your state regardless of your health status. You do not want to let this time pass you by.
There are a few states, Massachusetts, Minnesota, and Wisconsin, that standardize in a different way.
Here is the official Medicare definition:
Download the “Medicare and you 2022” handbook