The importance of the back-and-forth movement between data and graphs (or different graphs) is also indicated by the research on symbolizing. Steinbring (1997), for example, distinguishes reference systems and symbol systems. Students interpret a symbol system in the light of a better-known reference system. Reference systems are therefore relatively well known and symbol systems relatively unknown. In learning the relationship between a symbol system and a reference system, students must go back and forth between the two systems. A next step can then be that students use the symbol system they have just learned to reason with (Minitool 1, for example) as a reference system for a new symbol system (Minitool 2, for example), and so on.