When we think of pyramid we remember the conical shape, where the sides are bounded by triangular surfaces. We see some of the arches are triangular in shape. Triangle is a smallest polygon having three sides. It is a plane figure which is closed. We know that any polygon of more than 3 sides can be split into small triangles. We can form new shapes using triangles of same shapes. In this section let us discuss about the area and perimeter of a triangle, classification of triangles according to their sides and angles, properties of triangles and some problems based on properties of triangles.