Geographical factors in general and proximity in particular have a pervasive influence on negative as well as positive interaction between states. Traditionally, proximity has been measured by contiguity or by great-circle distance. We argue that it is important to distinguish between the different levels of interaction opportunities open to contiguous states and propose to operationalize this as the length of the shared land boundary. Data on boundary lengths can be used, e.g., in the study of the relationship between natural resources and conflict or in the study of the diffusion of conflict. To date, such datasets have only been available for the most recent years. We present a new dataset covering the entire Correlates of War period, explain the measurement procedures, and offer comparisons with other more limited datasets.