This paper documents an explorative case study aimed at investigating the knowledge and ideas held by myself, as a teacher, and those of students when working with the complicated and often confused historical second order concept codified as ‘similarity and difference’ in the latest History National Curriculum. This case study centres around a seven-lesson enquiry exploring the variation in lived experiences of several different persecuted groups of people during the Holocaust. This paper argues that whilst the ideas and application of similarity and difference as a concept from myself and students were widely divergent, students chose to argue conceptually in several different ways. In observing and defining the different forms of conceptual argument students engaged in, I am able to draw tentative ideas about progression when arguing conceptually with similarity and difference.