16 January 2015 Last updated at 22:30 GMT

How legal tide turned on same-sex marriage in the US

Same sex marriage is now legal in thirty-six US states, covering a majority of the US population.

A critical turning point came in October 2014, when the Supreme Court chose not to hear appeals against lower court rulings that had overturned same-sex marriage bans in five states.

That action was built on two years of court challenges, and many analysts predict it is only a matter of time before gay couples can marry across the country, especially after the US Supreme Court has said it will rule on the constitutionality of state bans in 2015.

So, what does it all mean and how did we get to this point?

How many US states allow same-sex marriage?

Thirty-six states are issuing marriage licences to same-sex couples, as well as Washington DC, which sets its own marriage laws but is not legally a state.

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. Also, in Michigan couples were briefly able to marry before a court stayed a ruling overturning its ban.

In many other states, same-sex marriage has been approved either through legislation, court rulings or voter referenda.

What about states where it is banned?

The remaining 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 several other states, although couples have not been wed there because the bans remain in place during appeals.

In November, the US Court of Appeals for the Sixth Circuit issued a decision that ran contrary to the wave of pro-gay marriage rulings and upheld bans in Ohio, Michigan, Kentucky and Tennessee.

What have been the key Supreme Court rulings?

On 6 October 2014, the court turned away appeals from five states with gay marriage bans on the books that had challenged appeals court rulings overturning those bans.

In challenging the gay marriage bans, proponents relied on a 2013 Supreme Court ruling in the case of United States v Windsor.

In that case, the court overturned the 1996 federal Defense of Marriage Act (Doma), which barred the federal government from recognising same sex marriages.

Under Doma, for example, 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.

Another key case, Hollingsworth v Perry of 2013, 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 is next?

In January 2015, the Supreme Court said they would hear the cases from the Sixth Circuit regarding marriage bans in four states, which created a dispute on this issue among the previously unanimous circuit courts.

The justices, who have previously have stopped short of resolving the question of same-sex marriage nationally, say they will consider whether or not states are constitutionally required to issue marriage licences and if states are required to recognise same-sex marriages performed elsewhere.

If the justices answer both of these questions in the affirmative, they will effectively fully legalise gay marriage in the US.