A $45\text{-}45\text{-}90$ triangle is a right triangle with two acute angles measuring $45^{\circ }$ and one right angle.

Since these two acute angles are congruent, its legs are also congruent. Therefore, the triangle is isosceles. If the legs have the length $\ell ,$ the length of the hypotenuse will be $\sqrt{2}\cdot \ell .$