If you sign up for parts A and B of original Medicare, also known as traditional Medicare, you can use any doctor or hospital that accepts Medicare payments. But you’ll still have out-of-pocket costs, such as deductibles, copayments and other expenses.

If you have retiree health insurance or Tricare military health care, that coverage may help fill in Medicare’s gaps. Otherwise, you can buy a Medicare supplement policy, better known as a Medigap policy, from a private insurer to help pay some of your out-of-pocket costs. If you choose a private Medicare Advantage plan, you won’t be eligible for Medigap.

Private insurers sell Medigap plans, but federal rules standardize coverage. Insurers can offer up to 10 different plans, each labeled with a letter. Plans with the same letter include the same benefits, even if different insurance companies offer them. And they can have different premiums.

All Medigap plans fully or partially pay for:

• Part A hospital coinsurance or copays.
• Part B coinsurance or copays for doctor visits and other outpatient services.
• 365 extra hospital days beyond Part A’s lifetime reserve days.
• First three pints of blood for transfusions.
• Hospice care coinsurance or copays.

Medigap plans may also help pay for:

• Part A deductible.
• Part A skilled nursing facility coinsurance. The first 20 days have no copay, but after that comes a $204 daily copay for each benefit period in 2024. Many Medigap plans cover all or part of that fee.
  
• Part B deductible, but this coverage is not available for those newly eligible for Medicare in 2020 or beyond.
• Part B excess charges. You may have to pay up to 15 percent more if your doctor charges more than the Medicare-approved amount.
• Cost of foreign travel emergency care up to plan limits.

What Medigap doesn’t cover:

• Long-term care.
• Prescription drugs, except for some plans sold before Jan. 1, 2006.
• Private-duty nursing.
• Routine dental exams, cleanings and X-rays.
• Routine eye exams and glasses.
• Routine hearing tests and hearing aids.
  
• Days in a skilled nursing facility beyond the 100 days per benefit period covered in Part A.

Even though each lettered Medigap plan must provide the same coverage, insurers can charge different premiums for a policy that has the same letter. Generally, the more generous the coverage, the higher the premium. Pricing also can be different based on these three methods:

Attained-age policies base premiums on your current age, so the premiums will rise as you get older. They also can increase because of inflation.

Community-rated policies charge the same premium regardless of your age.

Issue-age policies base premiums on your age at the time of purchase. Premiums can increase because of inflation and other factors but not because of your age.

Although the premium for an attained-age policy may initially be lower than the others, it will increase over time. Also, some pricing types are not available in all states.

A dot • below means a plan covers 100 percent of your out-of-pocket costs; a blank means you pay the costs. Note that average premiums are higher for men than women because Medigap insurers tend to pay more for their claims.

Carefully consider the advantages and disadvantages of signing up for original Medicare versus Medicare Advantage. If you have a health problem now — or develop one later in life — and you have coverage through Medicare Advantage, it may be difficult to buy a Medigap policy if you want to switch to original Medicare later on.

After you sign up for Medicare Part B at age 65 or older, you have six months to buy any Medigap policy in your area. If you want to sign up or switch policies after that, insurers in many states can refuse to sell you a policy or can charge you more because of preexisting conditions.

In certain circumstances, you have the right to buy a Medigap policy regardless of existing health problems, such as being enrolled in a Medicare Advantage plan that leaves the business or moving out of a plan’s service area.

When you’re enrolled in parts A and B of original Medicare, then explore your Medigap options — whether that’s during the seven-month initial enrollment period surrounding your 65th birthday or later.

The clock starts ticking on your six-month Medigap enrollment period— the first day of the month you have Part B at age 65 or older. During that time, you can’t be rejected for any Medigap policy sold in your area even if you have health problems, and you’ll get the best available rate.

You’ll find it’s not as easy to buy a plan later on. Insurers in most states can decide not to cover your preexisting conditions, charge you higher premiums or say no altogether.