Medicare Parts A and B work together to provide basic medical coverage, but they don't pay for everything. Medicare supplement insurance plans help pay for some of the out-of-pocket costs that Medicare Parts A and B don't pay. They do not include prescription drug coverage. Medicare supplement plans are offered by private insurance companies and work with the coverage provided through Medicare Parts A and B. They're identified by letters A - N and each standardized Medicare supplement plan must offer the same basic benefits, no matter which insurance company sells it. Cost is usually the only difference between Medicare supplement plans with the same letter sold by different insurance companies. (In Massachusetts, Minnesota, and Wisconsin Medicare supplement plans are standardized in a different way.) If you have a Medicare Advantage plan, you cannot have a Medicare supplement insurance plan.