When quantities have the same relative size. In other words they have the same ratio.
Example: A rope's length and weight are in proportion. When 20m of rope weighs 1kg, then:
• 40m of that rope weighs 2kg
• 200m of that rope weighs 10kg
Another example: The lengths of these two shapes are proportional: every matching side on the larger shapes is twice as large as on the smaller shape.