Alternative forms of security to prevent the arrest (or the continued arrest) of ships in admiralty proceedings in rem are vital in ensuring uninterrupted international trade. This paper examines one of the more popular and successful modern forms of alternative security, P & I club letters of undertaking. Club letters provide a fascinating example of highly effective private ordering by commercial parties within the broader framework of public judicial administration. Although undoubtedly contractual in nature, they have been required to respond and adapt to the particular requirements and policy concerns of the admiralty jurisdiction. It may be argued, therefore, that club letters have developed over time into a distinctive, if not sui generis, species of commercial surety.