(Reuters) - The following is a look at laws on gay marriage and same-sex civil unions in the United States after California's supreme court backed a ban on gay marriage.
* The first-same sex marriages in America took place in Massachusetts in 2004, a year after the state's highest court ruled that a ban on gay marriage was unconstitutional.
* Connecticut became the second U.S. state to legalize gay marriage when its supreme court overturned a ban on same-sex marriage on October 10, 2008. Local authorities began issuing marriage licenses on November 12.
* California began marrying gay and lesbian couples in June 2008, a month after the state Supreme Court ruled that barring same-sex marriages was unconstitutional. But that was reversed on November 4 when Californians voted in support of a proposition to amend the state constitution to ban same-sex marriage.
* Iowa became the third state to legalize same-sex marriage when its supreme court issued a unanimous ruling on April 3 that said a gay marriage ban violated the constitutional rights of gay and lesbian couples.
* Vermont followed suit on April 7 when lawmakers overrode a governor's veto of a gay-marriage bill, making the New England state the first in the country to legalize gay marriage with a legislature's vote.
* The District of Columbia city council voted on May 5 to recognize same-sex marriages performed in states where those unions are legal, although gay marriages cannot be conducted in the city itself.
* Maine's governor signed a bill on May 6 that cleared the way for the rugged state to become the sixth to legalize gay marriage. But the law could be delayed or even voided before it is due to take effect in September if opponents gather enough support for a statewide referendum on gay marriage.
* The New York State Assembly passed a bill on May 12 to legalize gay marriage but it faces an uncertain future in the state Senate, where Democrats hold a slim majority.
* A bill that would have made New Hampshire the sixth U.S. state to authorize gay marriage stalled unexpectedly on May 20 because of concessions to religious groups opposed to such unions. The bill has been sent to a committee where lawmakers from both chambers will try to resolve their differences.
* Forty-two U.S. states have laws explicitly prohibiting such marriages, including 29 with constitutional amendments restricting marriage to one man and one woman, according to the Human Rights Campaign, a gay rights advocacy group.
* New Jersey permits same-sex civil unions that grant largely the same state rights as married couples -- from insurance coverage to tax benefits and hospital visiting rights -- but lack the full legal protections of marriage.
* Hawaii and Washington each offer gay couples some legal rights as partners.
* The patchwork of laws has caused some unusual complications. Rhode Island's top court, for example, ruled in December 2007 that a same-sex couple married in Massachusetts could not legally divorce in Rhode Island, saying the state's family court did not have authority over same-sex marriages.
* The U.S. Supreme Court has not taken a case on gay marriage, leaving states to decide the issue. But eight same-sex couples who married in Massachusetts and three gay widowers filed a lawsuit on March 3 seeking access to federal protections and programs granted to straight married couples.
Compiled by Jason Szep in Boston; Editing by Bill Trott