Today, the Supreme Court moves on from Proposition 8 to the Defense of Marriage Act, which bans federal recognition of same-sex marriages. What's at stake in that case? Here's what you need to know.

What's the DOMA case about?

Thea Spyer (left) and Edith Windsor (right). (Bless Bless Productions)

United States v. Windsor concerns Edith Windsor, who was widowed when her wife Thea Spyer died in 2009. Windsor and Spyer were married in 2007 in Canada after being partners for 40 years. Windsor was forced to pay $363,053 in estate tax on Spyer's estate, which she argues she would not have to pay if she had been Spyer's husband. Thus, she claims, the Defense of Marriage Act, which prevents her from being considered Spyer's spouse for the purposes of federal taxes, literally cost her $363,053.

Who's the defendant?

Does this guy have standing to defend DOMA? House Speaker John Boehner (R-Ohio). (Larry Downing -- Reuters)

The Obama administration has declined to defend DOMA, and so the Bipartisan Legal Advisory Group (BLAG), a standing organization in Congress, took over the law's defense at the instruction of House Speaker John Boehner (R-Ohio). The U.S. District Court for the Southern District of New York ruled in June that DOMA's definition of marriage as between a man and a woman lacked a rational basis and ordered damages of $363,053 paid to Windsor. In October, the Second Circuit Court of Appeals concurred, with a panel ruling 2-1 for Windsor. Now the case goes to the Supreme Court.

What issues does the Court have to decide on there?

Supreme Court Associate Justice Anthony Kennedy, who is the likely swing vote in these cases. (Damian Dovarganes -- AP)

Three. The first is the equal protection issue, which is much the same in content as in the Proposition 8 case. The second is whether the fact that the executive branch agrees with Windsor means that there isn't a real controversy in this case, meaning the court doesn't have jurisdiction. The third is whether BLAG would be harmed by DOMA being overturned, and thus whether it has standing to defend the law (a friend-of-the-court brief by Harvard professor Vicki Jackson argues that even Congress doesn't have standing, and even if it did, BLAG wouldn't).

A ruling that focused on the second two issues would affirm the lower courts' rulings and effectively void DOMA only in New York, or other states where courts have ruled it unconstitutional. But a substantive ruling would void DOMA everywhere.

This post is an excerpted version of yesterday's "Everything You Need to Know about the Supreme Court's same-sex marriage cases."

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