There are two possible answers - that depend on the context.
The second definition is sometimes used by airlines for cabin luggage. The outside shape of the case matters, not how big it is inside.
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A diagonal is a line so the area of any diagonal must be zero.
A diagonal cannot be a side of a rectangle, and a side cannot be a diagonal.
Kitty-corner is another term for diagonal. A right triangle's hypotenuse is on a diagonal. On a keypad, the five is on a diagonal from the nine.
Use Pythagoras: Diagonal² = √(2 × sidelength²) → diagonal = side_length × √2 → diagonal = 12 × √2 ≈ 17.0 units
Using Pythagoras: diagonal² = side² + side² = 2 × side² → side² = diagonal² ÷ 2 area = side² = diagonal² ÷ 2 → diagonal² = 2 × area → diagonal = √(2 × area) = √(2 × 36) = 6√2 ≈ 8.49