The concept of symmetry is vital for the study of science and developing spatial and geometric relationships. Yet few studies have been conducted to examine how students learn this concept. This study investigates 757 Year 4 to 10 students' ability to reason about symmetrical relationships. By analysing the semiotic processes through the lens of the theory of semiotic mediation and Sfards' mathematical discourse framework, the results show the synergic relations between visualisation of artefacts, and the use of signs and keywords in the construction of knowledge.