This article draws an analogy between prime and irrational numbers with respect to how these numbers are defined and how they are perceived by learners. Excerpts are presented from two research studies: a study on understanding prime numbers by pre-service elementary school teachers and a study on understanding irrational numbers by pre-service secondary school teachers. Considering the results of these studies, the author calls for further attention in teaching to transparent features in representation of numbers and suggests several strategies on how this may be achieved.