What is true of the two diagonals of a rectangle?

Erklärung

Each diagonal closes off a right-angled corner whose other two sides are one length and one width of the rectangle, and the second diagonal closes a corner built on that very same pair of side lengths, so the two halves match and their diagonals are corresponding parts of equal size. Expecting them to meet at 90 degrees borrows a property from a shape with four sides of one length, and in a long thin rectangle the diagonals plainly cross at a shallow angle.