By Dominic Casciani
BBC News community affairs
What is a civil partnership - and what does it mean for gay couples in the UK?
Germany introduced similar equality legislation in 2001
What is a civil partnership?
Civil partnerships are a legally recognised union between two people of the same sex. While this sounds fairly dry and technical, it is to all intents and purposes a right of marriage for gay and lesbian couples.
Couples who become partnered will have the right to exactly the same legal treatment across a range of matters as a married couple would expect.
Such as what?
All the key elements that come with marriage. The law accords civil partners equitable treatment for important financial matters, such as inheritance, pensions provision, life assurance and maintenance where children are involved.
It provides next of kin rights for couples, such as in their dealings with hospitals. Immigration and nationality rules take account of marriage when assessing someone's right to stay in the UK - that rule is extended to civil partners.
Why is it not called gay marriage?
On a technical level there are differences. A partnership is formed when the second of the two parties signs the partnership papers. This is not necessarily a public ceremony or even an event that happens at the same time as the first signature.
This flexibility means that couples can essentially become partnered in private, if they wish. In contrast, a marriage happens when the partners exchange spoken words and also sign the register.
Another important distinction is that marriage as a word has religious connotations, even if a ceremony is only civil. Marriages can be conducted by Church of England clergy without any civil preliminaries being required. Civil partnerships are only conducted by registrars.
There are some key similarities: couples need to give public notice of their intention to partner in the same way as a man and woman who are marrying. The record of the partnership is also an official public document kept by the
General Registry Office in England and Wales (and its equivalents in Scotland and Northern Ireland).
There is, of course, a very large political reason why this is not officially gay marriage. Ministers are very careful to avoid using this kind of shorthand - it's almost certain that had they wanted to use such language in the legislation, the Act would have had a far harder time. They stress partnerships are an entirely new legal concept.
But beyond that are there any practical differences?
The government says that it wanted civil partnerships to provide as far as possible the same rights and responsibilities as for a married couple. Campaigners who advocated the change said they were more interested in getting parity for same-sex couples, which they achieved, than the terminology.
Clearly it's going to be a matter of personal taste. Among the first couples planning a partnership, some are referring it to "gay weddings" while others are not.
What does the government mean by responsibilities?
Civil partners will have duties in every way the same as married couples: Children within the family will need to be looked after; the second partner will be able to seek parental responsibility, similar to step-father/mother arrangements in heterosexual families.
A partner will be expected to provide reasonable maintenance and be part of the process of assessing a couple for benefits, if necessary. Crucially, the union does not end if someone walks away; civil partners will be required to go through a court-based dissolution which will address the same issues as any divorce settlement.
How many partnerships are predicted?
A range of major city councils have told the BBC that they have 1,200 bookings already and the government expects 4,500 by the end of 2006. Beyond that it is difficult to estimate, although ministers think between 11,000 and 22,000 by 2010.
Who opposes the law?
There are many people who still oppose the law, particularly some religious leaders and politicians.
At least two councils investigated whether or not they could have some kind of ban on civil partnerships, if only in preventing couples using their facilities. Both backed down amid the prospect of extremely expensive court battles that lawyers said they would lose.
Some registrars have signalled their opposition, according to newspapers, although they have a legal duty to conduct partnerships.
One religious group has however taken a different view. Rabbis with the Liberal Jewish tradition have said they will conduct services for gay couples. Figures from other religions have also already been involved in "blessings" for gay couples who have held "commitment ceremonies" before the law changed.
Are there implications for common law relationships?
At the moment, the so-called common law marriage actually has no legal force at all if things go wrong - and ministers are keen to point this out to couples who believe they do not need to marry to gain rights.
The Law Commission is however looking at the entire field, given that so many people no longer marry.