Triangles can be classified based on side lengths as equilateral, isosceles, or scalene triangles and based on angles as acute, obtuse, or right triangles. The three main properties of triangles are the angle sum property, exterior angle property, and Pythagorean theorem. Secondary parts of a triangle include the median, altitude, perpendicular bisector, and angle bisector. Triangles can be proven congruent using the SSS, SAS, ASA, AAS, or RHS criteria. Inequalities in triangles relate longer sides to larger angles and shorter sides to smaller angles. Important centers of a triangle include the incenter, circumcenter, centroid, and orthocenter.