"hook" and "thigh"of the two sides of the carpenter or try square. In the 3rd century, Zhao Shuang's commentary on the Zhoubi included a diagram effectively proving the theorem for the case of a 3-4-5 triangle, whence it can be generalized to all right triangles. The original text being ambiguous on its own, there is disagreement as to whether this proof was established by Zhao or merely represented an illustration of a previously understood concept earlier than Pythagoras.
The "hook" and "thigh" parts of a carpenter's or try square are discussed in the 3rd century commentary of Zhao Shuang on the Zhoubi, which includes a diagram that proves the Pythagorean theorem for a 3-4-5 triangle. This proof can be generalized to all right triangles, but there is debate about whether Zhao actually proved it or if he merely illustrated an earlier concept.