For any parallelogram, the complements of the parallelograms about the diagonal are equal to one another. Let $ABCD$ be a parallelogram, and $AC$ its diagonal. And let $EH$ and $FG$ be the parallelograms about  $AC$, and $BK$ and $KD$ the so-called complements (about $AC$). I say that the complement $BK$ is equal to the complement $KD$. <prf> Thus, the remaining complement $BK$ is equal to the remaining complement $KD$. Thus, for any parallelogramic figure, the complements of the parallelograms about the diagonal are equal to one another.