The concept of similarity bears close resemblance to the concept of congruence. Congruent figures are exact replicas of each other. They have the same shape and the same size. Now consider figures that have the same shape but not the same size. Such figures look 'similar' but in essence are simply proportionate to each other.

The concept of similarity has tremendous applications. In constructing a bridge or a building or a tower first the actual design is scaled down to the size of the paper. The actual structure and the design on paper have the same shape but not the same size. Here we introduce the concept of ratio and proportionality.