Triangles are three-sided polygons (closed figures) connected by points called vertices. Every triangle has three sides, which may be the same length--as in an equilateral triangle, or different lengths, as in isosceles and scalene triangles. 

If only two sides measure the same length, the triangle is isosceles. 

Just knowing the length of a triangle's sides tells us a great deal about the triangle. For instance, if we know that a triangle is equilateral, we know that it has other properties that we can use to solve problems. Of course, we are not always given the length of all three sides of a triangle. However, we can use what we know about the relationships among the sides of triangles to at least find a range of possible values for the missing information.