There are 12 standard medigap insurance policy types, called Plan A through Plan L. Any medigap policy offered by any insurance company must fall into one of these 12 plans, covering the same medical services as all other policies in the same plan. If someone doesn't already have a medigap policy, she's actually limited to only nine plans; that's because Plans H, I, and J used to offer prescription drug coverage and are no longer available to new buyers.

Also, in Massachusetts, Minnesota, and Wisconsin, some medigap plans have a few extra options. If a person lives in one of these states, the state's department of insurance and the insurance company offering the policy can explain that state's plan differences.

All medigap policies, in every plan type, include three basic coverage elements: They all cover the Medicare Part A hospital coinsurance amounts, plus 365 additional hospital days after Medicare coverage runs out. Every plan pays all or some of the Medicare Part B 20 percent coinsurance amount for doctor bills and other outpatient medical care. And every plan pays some or all of the cost of the first three pints of transfused blood, which Medicare does not pay.

This downloadable PDF shows the other Medicare "gaps" in payment covered by the various medigap plans. "Yes" indicates coverage of the gap and "No" indicates that coverage is not provided by that medigap plan. [Remember, Plans H, I, and J are no longer sold to new buyers.]