Definition
Double number line
A double number line uses two aligned scales to pair values from two related quantities.
View exact chart data
| Bags | Apples |
|---|---|
| 0 | 0 |
| 1 | 4 |
| 2 | 8 |
| 3 | 12 |
| 4 | 16 |
What it means
Read a double number line by looking across and straight down. Moving one equal step along the top line must mean the same top-line increase each time; the bottom line follows its own equal step. Marks that line up vertically belong together. This makes repeated pairs easy to see, such as 1 bag with 4 apples, 2 bags with 8 apples, and 3 bags with 12 apples.
Remember it
Look straight down from one scale to find its matching value on the other.
Example
Suppose every bag holds 4 apples. Put 0, 1, 2, 3, and 4 bags on the top line. Align them with 0, 4, 8, 12, and 16 apples on the bottom line. The mark under 3 bags is 12 apples, so 3 bags hold 12 apples.
Watch for
Two number lines placed near each other are not a double number line unless their aligned positions deliberately pair related values.
Common mix-up
The same physical gap does not have to mean the same number on both lines. Here one top step is 1 bag while one bottom step is 4 apples.
Exact meaning
A double number line is a pair of parallel, aligned number lines for two related quantities. On each line, equal horizontal gaps represent equal increments for that quantity, and vertically aligned marks show corresponding values. In a proportional relationship, every pair has the same multiplicative relationship.