This contribution relates to the secondary-tertiary transition and to the new flexibility requirements (changes of frames or registers...). We especially study the teaching and learning of complex numbers, at the beginning of the university cursus. First, we present experimental teachings using the online basis of exercises WIMS, to work on different representations of complex numbers (Rennes, France). Secondly, we analyse students’ productions working on complex numbers inside a geometrical frame (Namur, Belgium). It seems that students willingly move from a geometrical to an algebraic frame, but the choice of the most appropriate one remains a challenge.