A Medicare supplement, also known as a Medigap plan, fills in some of the gaps in your Medicare hospital (Part A) and medical (Part B) coverage. When you receive Medicare-covered services, Medicare pays first, then the supplement pays some or all of the remaining costs. The nice thing about a supplement is that there are no network restrictions – you can go to any doctor or hospital that accepts Medicare, and no referrals are required.
There are ten standardized Medicare supplement plans, which means that a Plan F from one insurance company, for instance, will cover the same expenses as a Plan F from another insurance company. When you first sign up for Medicare Part B, regardless of age, you have 6 months to purchase a Medicare supplement without answering any medical questions; after that, any application or changes you want to make may require medical approval.