Americans aged 65 years or older and eligible for Social Security can enroll in Medicare. Original Medicare, defined as Medicare Parts A, B and D, covers expenses relating to services and supplies considered medically necessary, such as hospital visits, nursing facility care and some prescription drug costs.

However, Original Medicare does not provide fully comprehensive coverage, and many Americans elect to purchase supplemental insurance. If you’re currently enrolled in Medicare it’s important to know how Medicare Supplemental insurance works and how it may benefit you:

Help with expenses
If you’re already enrolled in Medicare Parts A and B, you can purchase a Medicare Supplemental  insurance policy from a private insurer. Sometimes called Medigap policies, this insurance may help you avoid some of the health care costs that Original Medicare doesn’t provide for. This may include co-payments, co-insurance, deductibles and other out-of-pocket expenses.

Avoiding coverage gaps
While Original Medicare is designed to ease the health care cost burden of older adults, it still has many coverage gaps. As U.S. World News and Report explained, while preventative care services are free under Medicare, doctors may sometimes need to order follow-up tests during annual screening. Additional testing is usually not considered preventative care and will not be covered. Additionally, there is no upper limit on how much beneficiaries may have to pay out of pocket with Original Medicare. One serious injury or illness could result in thousands of dollars in medical expenses.

The freedom to customize
As CBS News explained, Medicare Supplemental Insurance is offered through 10 standardized plans, which are regulated by the federal government. Each policy has the same basic coverage, but some have additional benefits as well. Not all states will offer all these plans and not all insurers offer coverage for each type. However, the variety of options means you can compare policies and find the plan that best suits you, while also comparing premiums across different insurance providers.

Low monthly premiums
Some plans also offer high-deductible variations that have lower monthly premiums. This can be an attractive option for healthy seniors who can afford some out-of-pocket expenses and don’t expect to use their medical insurance very often.

Coverage during foreign travel
If you’re like many seniors, you might be planning to spend a significant portion of your retirement traveling. But before you hit the road, you should make sure you understand how travel affects your health care coverage. In most cases, if you’re only enrolled in Original Medicare, you will not be covered for any medical expenses that occur outside the U.S., meaning the 50 states, the District of Columbia, Puerto Rico, the U.S. Virgin Islands, Guam, American Samoa and the Northern Mariana Islands. If you plan to spend your retirement seeing the world, adding a Medicare Supplemental insurance policy may provide up to 80 percent coverage of any foreign medical expenses you have after you meet your deductible.

If you decide to purchase a Medicare Supplemental insurance policy be sure to talk to your insurance agent about your existing medical needs and plans for your retirement. This will help the agent to guide you to the best Medigap plan for you. The Centers for Medicare and Medicaid Services also offers easy comparison tools through the official Medicare website.