Short answers, pulled from the story.
Denmark was the first country to legalize civil unions. The law was introduced on the 7th of June 1989 and came into effect on the 1st of October 1989, establishing registered partnerships for same-sex couples.
Civil unions grant some or all of the rights of marriage but do not carry the title of marriage. Common exceptions include child adoption rights, immigration benefits, and federal tax treatment. In the United Kingdom, civil partnerships require their own dissolution process rather than divorce.
Vermont introduced the first civil unions in the United States in 2000. The law was a response to the Vermont Supreme Court ruling in Baker v. Vermont, which required the state to grant same-sex couples the same rights as married couples under state law.
The Pacte civil de solidarité, or PACS, is France's civil union framework, introduced in 1999. Unlike many civil union laws, it is available to both same-sex and opposite-sex couples and dissolves more easily than a marriage. By 2019, ninety-six out of every hundred PACS couples in France were heterosexual.
Critics argue that civil unions create a separate but unequal status, excluding same-sex couples from the right to marry while forcing them into a different legal institution. A New Jersey commission found that the state's civil union law invited and encouraged unequal treatment of same-sex couples and their children. Advocates also note that civil unions are often not widely understood, which can cause practical difficulties in emergencies.
In most countries that established same-sex civil unions, the laws were later replaced by same-sex marriage. Denmark's registered partnership law ran from 1989 to 2012, Germany's from 2001 to 2017, and Switzerland's federal partnership law from 2007 to 2022. In the United States, by October 2014, every state offering civil unions also allowed same-sex couples to legally marry.