This study explores how assessment criteria are applied in grading student work. It is found that explicit assessment criteria do not work as authoritative guidance as expected and that tacit criteria are more decisive in awarding a certain grade. Various sources that form idiosyncratic tacit criteria are identified. These sources, including values on curriculum and assessment, teaching experiences, and the way of supervising grades, work as different social contextual elements that influence judgements on grading students' work. The study suggests that tacit criteria and the different sources that form the tacit criteria need to be identified, perceived, and communicated in the community of practice to reduce grade variability and achieve a shared understanding of grading.