For a right triangle with legs and hypotenuse , the squares on the two legs have the same combined area as the square on the hypotenuse. The right angle is essential; arbitrary triangles do not obey this identity.直角三角形的两条直角边为 ，斜边为 。以两条直角边为边长的正方形，面积之和等于斜边上正方形的面积。直角是必要条件，一般三角形不满足这个等式。
The green and cyan squares sit on the legs; the gold square sits on the hypotenuse. The legs change while the angle stays right. Below, the green and cyan area bars together always match the gold bar at the same scale.绿色和青色正方形分别对应两条直角边，金色正方形对应斜边。边长变化时，夹角始终保持直角。下方使用同一比例尺：绿色与青色面积条拼接后的长度，始终等于金色面积条。