Beneficiaries who purchase a Medicare supplement plan pay an additional monthly premium on top of their Part B premium to a private insurance company approved by Medicare. Medicare does not pay any of the costs for a Medigap policy.
All Medigap policies have standard features, no matter which company is marketing the plan, but each has somewhat different healthcare coverage. The most popular is Plan F, which includes full co-payment for Part B. This will cover such things as the costly cancer drugs administered in a doctor's office or a hospital outpatient facility. There are 10 plans identified by letters.
Here is Medicare's description for consumers: "Every Medigap policy must follow federal and state laws designed to protect you, and the policy must be clearly identified as 'Medicare Supplement Insurance."
Medigap insurance companies in most states can only sell you a 'standardized' Medigap policy identified by letters A through N. Each standardized Medigap policy must offer the same basic benefits, no matter which insurance company sells it. Cost is usually the only difference between Medigap policies with the same letter sold by different insurance companies.