Although the Supreme Court is a singular institution within the American judiciary, it remains recognizably a court. Among the many institutional characteristics that mark the Court as a court, perhaps the most defining is that it exercises its law-making function via justificatory opinions that accompany dispositions in individual cases. By conforming its lawmaking texts to the judicial genre, the Court distinguishes itself from other government actors, such as Congress and the president, that exercise government authority through other textual forms, such as statutes and executive orders. This Article presents the results of a quantitative analysis of Supreme Court opinions that measures the degree to which the content of the Court’s opinions conforms to, or departs from, the judicial genre. With the opinions of the federal appellate courts as a baseline, we use topic modeling to estimate the degree of semantic distinctiveness of Supreme Court opinions and track changes in that distinctiveness over the second half of the twentieth century. We find that the Court has become measurably more distinctive over time. We further find that the divergence of the Court’s opinions from the judicial genre is not due to the selection of an increasingly non-representative pool of cases for review. Rather, the Court is analyzing and writing about a similarly representative pool of cases, but in an increasingly idiosyncratic fashion. We accompany this quantitative analysis with a qualitative analysis of individual Supreme Court opinions that draws out the significance of this change.