Sanders’s bill takes universal coverage close to this extreme: The government insurance would cover so many services with such small co-pays that private insurance would be almost universally unnecessary. Accordingly, it would also be quite expensive — $32 trillion over 10 years, according to an Urban Institute report. That’s more than a 50 percent increase in federal spending — all federal spending — according to spending projections by the Congressional Budget Office. That would be partially offset by people no longer needing to pay premiums to private insurers, however, and the government’s monopoly could allow it to implement cost-saving measures.

But most universal-coverage systems don’t look quite like this. It’s expensive for a government to fund a comprehensive health-care system, especially somewhere like the United States, where costs are so high.

So many governments instead pay for most but not all of their residents’ medical treatment. In those countries, people have the option to buy “supplementary” private insurance, which pays for services such as dental care that the government health-care program excludes. People often also have the choice to buy “complementary” private insurance, which pays for the co-pays and deductibles in the government’s insurance plan.