A number line, is a straight line with evenly space marks on the line and the marks are uniformly numbered along the entire line. The number line below shows numbers from -10 to 10, with the negative numbers to the left of zero and the positive numbers to the right of zero. Note that the arrows at each end of the number line indicate that the numbers continue on forever in each direction.

The number line is usually drawn as a horizontal line. The x-coordinate, used to indicate the measure left or right in graphing, is a number line. However, a number line can also be drawn vertically, with the positive numbers above zero and the negative numbers below zero, similar to a thermometer. The y-coordinate, used to indicate the measure up and down in graphing is also a number line.

Although the number line shown above only shows whole numbers, a number line represents all real numbers. For example, 3.5 would be half way between 3 and 4, as shown below.

Number lines can be helpful in understanding addition and subtraction.  Let's determine what happens when you add eleven to minus four (-4 + 11).  Looking at the number line below, start at -4, which is marked in green, and move right 11 units, which yields the answer, 7.  Note the green starting point (-4) is marked like a green light (GO) and the ending point (7) is marked like a red light (STOP).  Since we added a positive number, we moved right.  If we had added a negative number, we would have moved left, instead, which is subtraction.

Let's determine what happens when you subtract fourteen from five (5 - 14).  Looking at the number line below, start at 5, which is marked in green, and move left 14 units, which yields the answer, -9.  Since we subtracted a positive number, we moved right.  If we had subtracteded a negative number, we would have moved right, instead, which is addition.

Note that if you mark any point (number) on the number line, all other numbers to the right of that mark are greater, or larger, than the marked number and any number to the left of the mark is less than or smaller than the marked number. To illustrate this point, note how 2 is greater than -4, on the number line below. Likewise, -4 is less than 2.