When measuring the length of both diagonals of a parallelogram, one can notice that the two diagonals are of equal length. This property is a consequence of the fact that opposite sides of a parallelogram are parallel, which creates two pairs of congruent triangles when the diagonals are drawn. By the Side-Side-Side (SSS) congruence criterion for triangles, these pairs of triangles are congruent, and therefore their corresponding sides, including the diagonals, are of equal length.