The major legal treatises of the Kingdom of Jerusalem all date from the thirteenth century, after the kingdom had largely been reconquered by its Muslim neighbours. There are no comparable sources for the first period of the kingdom in the twelfth century. The kingdom, evidently, did have a legal system in the twelfth century, but was it anything like the system described by the thirteenth-century texts? Is it possible to know what laws were being used, and the origins of these laws? Twelfth-century chronicles, charters, and other sources occasionally mention the courts and particular cases, but they are usually vague and lacking in detail. Of these, the most useful source is Usāma ibn Munqidh, whose observations of Frankish customs are often dismissed as unreliable jokes. However, his descriptions of crusader justice in the twelfth century are in fact surprisingly accurate when compared to the thirteenth-century legal texts. This shows that at least some of the laws and administrative offices described in the thirteenth century were already in place by the middle of the twelfth, and allows us to identify more precisely the possible influences and origins of the laws.