## Introduction

The number line is featured prominently in the Common Core Content Standards for Mathematics (CCCSM) as a model for representing numbers. It first appears in the Measurement and Data domain in second grade wherein students are expected to represent numbers as lengths on the number line as well as sums and differences on a number line. Number line references are numerous in grade three, particularly in the Number and Operations domain, but in Measurement and Data as well. Reliance on the number line continues in grade four; references are seen through grade eight as well as in the high school Statistics and Probability domain.  Not only does use of the number persist across grade levels, but also across domains.

• The number line serves as a visual /physical model to represent the counting numbers and constitutes an effective tool to develop estimation techniques, as well as a helping instrument when solving word problems.

• The number line

constitutes a unifying and coherent representation

for the different sets of numbers (N, Z, Q, R) which the other models cannot do.

• The number line is an appropriate model to make sense of each set of numbers as an expansion of other and to build the operations in a coherent mathematical way.

• The number line enables to present the fractions as numbers and to explore the notion of equivalent fractions in a meaningful way.

• The number line, in some way, looks like a ruler, fostering the use of the metric system and the decimal numbers.

• The number line fosters the discovery of the density property of rational numbers.

• The number line provides an opportunity to consider numbers that are not fractions and consider the existence of irrational numbers.