Definition: A number line is a straight line on which every point matches a number, with equal spacing between marks and values increasing from left to right.
What is a number line?
A number line is a picture of numbers laid out in order. Zero sits at the origin, positive numbers go to the right, and negative numbers go to the left. Equal spacing between tick marks means equal differences in value.
How do you place numbers on it?
Choose a scale first. If each tick is 5 units, then the tick after 10 is 15. To plot -35, count seven ticks left from zero. Fractions and decimals fit between the integers: 2.5 lies halfway between 2 and 3.
How does it help with order?
The farther right a number is, the greater it is. So -1.5 is greater than -4 because it is to the right. You can read inequalities straight off the line.
How does it help with distance?
The distance between two points is the number of units separating them. From -6 to 9, the distance is 6 + 9 = 15 units.
What is the common mistake?
Using uneven spacing. If the ticks are not equal, the line misleads you. Always check that the gap between 0 and 1 matches the gap between 1 and 2.