Jan Wessel initially analyzes subtraction from a theoretical perspective. He highlights the concepts of remainder and difference as fundamental ideas of subtraction. Building on this, he empirically investigates to what extent first-grade students utilize difference concepts and related strategies to solve subtraction problems. Based on an interview study, he demonstrates that the use of these fundamental concepts occurs on a spectrum between operational and relational realization, and that children perform both explicit and implicit shifts in their fundamental concepts when solving subtraction problems.