A single, fairly simple proposition lies at the heart of the present contribution, viz. that the development of early-modern literature on international law should be regarded as a specific form of the early usus modernus during the second half of the sixteenth century and the first half of the seventeenth century.In Section 1 that general proposition and some of its ramifications will receive some further explanation. First, the main characteristics of usus modernus will be set out, and, subsequently, their applicability to early-modern literature on international law will be outlined. In addition, the question should be raised, what purpose the characterisation of that literature as part of the more general usus modernus may serve. These considerations may inevitably appear somewhat abstract. Section 2 will therefore aim to provide a particular illustration, based on two contemporary works on the law of naval warfare, viz. A. Gentili's Hispanicae Advocationis Libri Duo and H. Grotius' De jure praedae.