We see many structures, objects figures etc. that are evenly balanced and in pleasing proportions, by merely having a look at them we get to know that they are symmetrical. Some of the live examples are the Taj Mahal and India Gate etc. These structures are beautiful because of their symmetry.
An object or a structure when cutting into two halves giving the same structure of equal length is called symmetry, and the line which passes through them is the line of symmetry.
Symmetry Exercise 17.2