Similarity (general):

Two plane figures are called *similar* if an *enlargement* of one figure is *congruent* to the other.

That is, if one can be mapped to the other by a sequence of *translations*, *rotations*, *reflections* and *enlargements*.

Similar figures thus have the same shape, but not necessarily the same size.

Similarity (triangles):

There are four standard tests to determine if two triangles are similar

**AAA:** If two angles of one triangle are respectively equal to two angles of another triangle, then the two triangles are similar.

**SAS:** If the ratio of the lengths of two sides of one triangle is equal to the ratio of the lengths of two sides of another triangle, and the included angles are equal, then the two triangles are similar.

**SSS:** If we can match up the sides of one triangle with the sides of another so that the ratios of matching sides are equal, then the two triangles are similar.

**RHS:** If the ratio of the hypotenuse and one side of a right-angled triangle is equal to the ratio of the hypotenuse and one side of another right-angled triangle, then the two triangles are similar.