A triangle is a polygon with three edges and three vertices. It is one of the basic shapes in geometry.
Triangles are classified by sides: equilateral (all equal), isosceles (two equal), and scalene (all different). By angles: acute (all < 90°), right (one = 90°), and obtuse (one > 90°). The sum of angles is 180°. Triangles are rigid shapes, making them essential in construction (trusses, bridges).
Triangles were studied extensively by Greek mathematicians. Euclid devoted Books I-IV of his "Elements" to triangle geometry. Pythagoras proved his famous theorem about right triangles. Thales used triangle properties to measure the height of pyramids.