Through the institution of civil marriage all countries in Europe recognise regulate different-sex couples. As a legal institution marriage can be characterised as a form of partnership between two persons that is created by a formal act of registration, and that results in a number of legal consequences (rights and obligations, both between the partners, and between the partners and others including the state). Since the 1970s a growing number of European countries have made a growing number of these legal consequences available to unmarried partners in informal cohabitation. This legal recognition of informal cohabitation has sometimes been restricted to different-sex couples, while sometimes same-sex couples have been included. Since 1989 several European countries have introduced registered partnership, a legal institution that is more or less analogous to marriage, resulting in some or almost all of the legal consequences of marriage. In some countriesregistered partnership has only been made available to same-sex couples, while others made it also availableto different-sex couples. And since 2001 a few European countries have opened up civil marriage to same-sex partners. With all these developments, the field of ‘family law’ (in the wide sense of the word) has become much more complex and varied (and ‘same-sex-friendly’) than it used to be. Even lawyers rarely have a comprehensive understanding of the differences between the marriage, registered partnership and cohabitation in their own country, let alone in other countries. Over the next few years these developments will become evident in more countries. Therefore it is becoming simultaneously more interesting and less easy to analyse this field of law. The challenge is how to carry out comparisons in at least five ‘dimensions’: between marriage, registered partnership and cohabitation, between different-sex and same-sex partners, between different areas of private and public law, between different countries, and between now and previous years or decades. The present study introduces a tool for such a complex comparative analysis. The tool is called ‘level of legal consequences’ or ‘LLC’. That tool is applied here to the nine European countries that by 2003 had introduced a form of registered partnership at national level: Belgium, Denmark, Finland, France, Germany, Iceland, Netherlands, Norway and Sweden. This study is the result of the cooperation of nine lawyers in the multi-disciplinary project of the French National Institute of Demographic Studies (INED) on the phenomenon of registered partnership. That multi-disciplinary project also comprises sociologists, historians, statisticians and demographers. The results of their work are and will be published elsewhere.