In order to have a Medicare supplement, a person must also have coverage under Medicare Part A and Part B. The Medicare supplement picks up where Medicare leaves off. Medicare Part A (hospitalization) has a deductible and additional costs for extended hospital stays. Part B has a deductible and then generally covers 80% of medical costs. Medicare supplement coverage depends on which of the available plans you choose.
Medicare supplement coverage is preferred by many seniors because it allows them to go to any doctor or hospital that accepts Medicare (generally speaking) and and out of pocket costs are very small, especially if the Medicare supplement covers the Part A deductible.