The aim of this paper is to analyze the affordances and constraints of the way dynamic representations are used in the digital learning lab 'The set line' from the Dragonbox School teaching material. The learning lab is designed for second graders. The starting point of our analysis is the task 18 + 6. Our analysis shows that 'The set line' offers affordances for the users' learning of addition and other aspects of the number concept because the regrouping involved in the addition of 18 and 6 is explicitly displayed by the dynamic representations in the learning lab. 'The set line' may also cause constraints for students' opportunities to learn the inherent mathematical concepts and relations because several operations are carried out automatically by the learning lab.