Few people today are likely to concur with the classical Greek poet Pindar’s maxim, “Custom is the king of all.” Nevertheless, the thrust of this book is that custom – the unofficial and unenacted practices of communities – lives as a source of obligation in contemporary legal cultures and remains a potent jurisprudential force, for both domestic polities and international law.

A central puzzle in jurisprudence has been the role of custom in law. Custom is simply the practices and usages of distinctive communities. But are such customs legally binding? Can custom be law, even before it is recognized by authoritative legislation or precedent? Assuming that custom is a source of law, what are the elements of custom? Is proof of a consistent and long-standing practice sufficient, or must there be an extra ingredient that the practice is pursued out of a sense of legal obligation or, at least, that the custom is reasonable and efficacious? How does one actually prove a custom? Most tantalizing of all, is custom a source of law that we should recognize in modern, sophisticated legal systems, or is the notion of “law from below” outdated, or even dangerous, today?