Massachusetts was the first state to legalize same-sex marriage. Eight more states have followed. But much remains to be clarified, even for same-sex couples in Massachusetts and those other states, about what their rights are under federal law.

That is why today’s oral argument in the U.S. Supreme Court on same-sex marriage is so important. The Court is considering a challenge to the federal Defense of Marriage Act (DOMA). DOMA denies same-sex couples federal benefits that are available to people in heterosexual marriages.

One of those benefits is the estate tax exemption that is at issue in the case before the Supreme Court. The case is called United States v. Windsor. It involves a lesbian couple from New York who were partners for decades before finally being allowed to marry. When one partner died, the other inherited her estate.

The estate was a valuable one, worth about $4.1 million. But the amount that the surviving partner actually inherited was several hundred thousand dollars less than it would have been in a heterosexual marriage.

This was because of the Defense of Marriage Act. Under DOMA, federal law prohibits a surviving spouse in a same-sex marriage from receiving the same estate tax exemption that a surviving spouse in a heterosexual marriage would receive.

It is quite possible, however, that the Supreme Court will find DOMA to be unconstitutional. The problem is not merely the inheritance law at issue in Windsor, but over 1,000 other federal laws that provide benefits to heterosexual married couples but not same-sex married couples.

Source: “A User’s Guide to Day 2 of Gay Marriage at the Supreme Court, DOMA Edition,” The Atlantic, 3-27-13