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Similarity is an idea in geometry. It means that two polygons, line segments, or other figures have the same shape. Similar objects do not need to have the same size. Two shapes are similar if their angles have the same measure and their sides are proportional. Two circles, squares, or line segments are always similar.
Triangles are special in similarity. This is because triangles can be similar if only their angles are equal, or only their sides are proportional. All other polygons must meet both of the conditions.
Similarity is very similar to congruence. Congruent shapes have the same sides and angles. In fact, all shapes that are congruent to each other are also similar.