This article draws attention to communication across professions as an important aspect of forensic evidence. Based on ethnographic fieldwork in the Swedish legal system, it shows how forensic scientists use a particular quantitative approach to evaluating forensic laboratory results, the Bayesian approach, as a means of quantifying uncertainty and communicating it accurately to judges, prosecutors, and defense lawyers, as well as a means of distributing responsibility between the laboratory and the court. This article argues that using the Bayesian approach also brings about a particular type of intersubjectivity; in order to make different types of forensic evidence commensurable and combinable, quantifications must be consistent across forensic specializations, which brings about a transparency based on shared understandings and practices. Forensic scientists strive to keep the black box of forensic evidence – at least partly – open in order to achieve this transparency.