Q&A: Same-sex marriage in the US
Support for gay marriage in the US has gained momentum since the US Supreme Court delivered victories to supporters in two landmark rulings in June 2013.
Since then, same-sex nuptials have been approved in several states by legislation or through the courts. Other states have cases pending.
And in the last year, supporters of gay marriage have won more than a dozen consecutive court decisions.
So, what does it all mean and how did we get to this point?
How many US states allow same-sex marriage?
Gay weddings are legal in 19 states: California, Connecticut, Delaware, Illinois, Iowa, Maine, New Jersey, Maryland, Massachusetts, Minnesota, New Hampshire, New York, Oregon, Pennsylvania, Rhode Island, Vermont, Washington state, New Mexico and Hawaii, as well as in Washington DC.
In addition, same-sex couples married in Arkansas after a local judge ruled 1997 and 2004 bans unconstitutional, but marriages there were halted soon afterwards, pending an appeal of the case.
Such unions have been approved either through legislation, court rulings or voter referendums.
What about states where it is banned?
The remaining 31 states do not legally sanction gay marriage, but laws vary greatly. Some states have civil union laws that grant couples most if not all of the rights and benefits of marriage.
Other states have passed bans or state constitutional amendments forbidding such unions. Some ban same-sex marriage but recognise couples married in other states.
Bans on gay marriage have been quashed in recent months in Indiana, Michigan, Texas, Utah, Oklahoma and Virginia, although couples have not been wed there because the bans remain in place during appeals. In addition, appeals court decisions have called into question the future of gay marriage bans in West Virginia and North and South Carolina.
It is likely the question of gay marriage will return to the Supreme Court through these cases.
What has the Supreme Court already decided?
Describing marriage as a civil rights issue, advocates said that the US should not have a two-tiered system in which some people may marry - and receive the financial benefits that come with being married - while others may not, while others argued it should be determined by each US state.
Opponents of same-sex marriage adhered to what they call the traditional definition of marriage - between a man and a woman - and said the government should not redefine that.
The recognition of gay unions by some states, but not the federal government, created a legal conundrum that allowed advocates to bring the issue to the top court in the US.
The first case before the Supreme Court, United States v Windsor, was a challenge to the Defense of Marriage Act (Doma), passed by Congress in 1996. Under this law, same-sex marriages approved at the state level were not recognised by the federal government.
This meant, for example, that individuals in same-sex marriages were ineligible for benefits from federal programmes such as the Social Security pension system and some tax allowances if their partners died.
Edith Windsor, an 83-year-old widow, said it was unfair that she was required to pay $363,000 (£239,000) in extra taxes after her wife's death, simply because she was a woman.
The other case, Hollingsworth v Perry, was filed by two lawyers, Theodore Olson and David Boies, working together on behalf of their California clients, Kristin Perry and Sandra Stier and another couple, Jeffrey Zarrillo and Paul Katami.
They argued that the Supreme Court should strike down a state law, called Proposition 8, which stated that marriage is between a man and a woman. The law, approved by California voters in 2008, overrode a state Supreme Court decision that allowed for same-sex marriage.
What does the ruling on Doma mean?
The Doma decision means that legally married gay men and women are entitled to claim the same federal benefits available to opposite-sex married couples.
These cover areas like tax, social security and immigration.
The challenge to Doma did not address the question of whether same-sex marriage was constitutional.
What about Prop 8?
The decision, which effectively legalised gay marriage in California, was decided on technical grounds when the majority of judges, 5 to 4, decided the case was not properly before the court.
Supporters of Proposition 8, they ruled, were not entitled to appeal the decision by another court in California to strike down the ban.
So, the Supreme Court left in place the victory by the state court for two same-sex couples who had sought to marry.