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

• The number line serves as a visual /physical model to represent thecounting 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 ofnumbers as an expansion of other and to build the operations in a coherentmathematical way.

• The number line enables to present the fractions as numbers and to explorethe 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 ofrational numbers.

• The number line provides an opportunity to consider numbers that arenot fractions and consider the existence of irrational numbers.