The Supreme Court heard arguments this morning in a case that could threaten the viability of the Affordable Care Act, President Obama’s signature health care law. The law, also known as Obamacare, survived a previous Supreme Court challenge in 2012. But this new case, called King vs. Burwell, has many people worried, and rightfully so. At issue in the case are the financial subsidies provided to millions of Americans to buy health insurance through the insurance marketplaces, called exchanges. Without these subsidies, many people of lower income would not be able to afford coverage. Stopping these subsidies would put a big dent in the “affordable” part of the law’s name. The plaintiffs argue that the Affordable Care Act allows for subsidies to be provided when insurance is purchased on exchanges “established by the State” (there are 17 of these), but not on the exchange established by the federal government for the other states. The outcome of the case may hinge on how the justices interpret those four words in the health law.