The backbone of President Obama’s health care law is taking shape, with 26 states choosing to let the federal government run the online insurance markets mandated by his signature reforms instead of keeping the job in-house or partnering with the feds.

The Department of Health and Human Services had encouraged states to run their own markets, or “exchanges,” that help the uninsured find coverage. Only 17 states and the District of Columbia took on the task, while seven states decided to split the duty with the Obama administration, according to a breakdown by the Kaiser Family Foundation.

The exchanges, which are designed to let those without employer-based insurance compare and buy plans with the help of tax credits, are a crucial part of the Patient Protection and Affordable Care Act that passed in 2010 and largely was upheld by the Supreme Court in June. States that wanted to run a partnership exchange with the federal government had to let HHS know by late Friday, ending months or even years of debate among governors and state lawmakers.

Their discussions marked one of two major decisions under “Obamacare.” Whether or not to expand Medicaid within their borders is the other, and it remains a source of contention in state capitals across the country.