Cathetus and hypotenuse
Prerequisite for the Pythagorean theorem is a right triangle. Every right triangle always has a $90^\circ$ angle. If the triangle does not have a right angle, none of these phrases applies.
If the requirement is fulfilled, you have to determine the catheti and the hypotenuse first:
- The hypotenuse is the opposite side to the right angle. It is also the longest side in the triangle.
- Catheti are the other two sides that are not opposite to the right angle.