What do you call the longest side of a right triangle?

In a right triangle, the longest side is known as the hypotenuse. This designation comes from the geometric properties of right triangles, where one angle measures 90 degrees. The hypotenuse is always opposite the right angle and is longer than either of the other two sides, which are referred to as the adjacent side and the opposite side.

The adjacent side is the one that forms one of the angles in the right triangle, while the opposite side is the side opposite to the angle in question. The distinction is crucial in trigonometry, where the lengths of the sides relative to the angles are fundamental in defining various ratios. The vertical side terminology is not standard in this context and does not accurately describe any side in terms of length or relation to the right angle.

Thus, identifying the hypotenuse as the longest side underscores its role in the Pythagorean theorem, which states that the square of the hypotenuse's length equals the sum of the squares of the other two sides' lengths. This relationship further emphasizes why the hypotenuse stands out as the longest side in a right triangle.