The main concern of this chapter is the association of numbers with a visually represented line. The association is usually acquired in junior school, and is then extended to provide an integrated representation of integers, fractions, and irrational numbers, called the real number line. However, the nature and origin of mental number lines, and the way they are embedded in our thinking, are not obvious. This chapter discusses mental number lines given the evidence to date. It shows how innate and cultural factors interact to determine the nature and role of mental number lines in basic numerical thinking. It also underscores their importance in more advanced mathematics.
Keywords: mental number lines, visual thinking, number size, number comparison, number sense, SNARC effect