A civil union, also referred to by a variety of other names, is a legally recognized arrangement similar to marriage. These unions have been established in a number of countries since the late 1990s, often developing from less formal domestic partnership legislation. In Denmark, Iceland, Norway and Sweden, they have since been replaced, and in a number of other countries supplemented, by same-sex marriage. Civil unions are often seen by campaigners as a "first step" towards legalizing marriage for same-sex couples. While civil unions are predominantly established for both opposite-sex and same-sex couples, in a number of countries they are available to same-sex couples only.