We live our lives against an extensive backdrop of legal rights and responsibilities, yet a growing number of studies indicates low levels of public legal literacy. In the context of opposite-sex cohabitation and marriage law, this study employs new survey data from the United Kingdom to explore, in detail, how many and which people are ignorant of the law, and what are the nature and origins of erroneous beliefs. We find that people's beliefs about both cohabitation and marriage law are frequently wrong. They are also strikingly similar, and reflect the divergence of social attitudes from the law. Our findings are consistent with the notion that legal literacy links to salience of issue. They are also consistent with recent public legal education initiatives that affected public understanding of cohabitation law, but we argue that social attitudes and the intransigence of erroneous beliefs generally present significant challenges to such initiatives.