State supreme courts frequently cite each other as authorities. These citations constitute the interstate communication of precedent. The major direct predictors of this inter-court communication are the legal capital of the cited court, the difference in legislative innovativeness between the two states, and contemporaneous interstate migration. After 1940, inter-court communication is more strongly related to interstate migration than to any other predictor in the analysis. The relationship between interstate migration and inter-court communication suggests that the interstate communication of precedent is affected by cultural regionalism. Inter-court communication is not increased by similarity between states in population size or in levels of urbanization and industrialization. Courts of populous and urban states are cited more by courts of less populated and rural states, but these relationships are mediated by differences in legal capital, legislative innovativeness, judicial professionalism, and interstate migration. Judicial professionalism and the West's regional reporter system are directly related to inter-court communication in the later years surveyed.