In parallelogrammic figures the opposite sides and angles are equal to one another, and a diagonal cuts them in half.Let $ACDB$ be a parallelogrammic figure, and $BC$ its diagonal. I say that for parallelogram $ACDB$, the opposite sides and angles are equal to one another, and the diagonal $BC$ cuts it in half. <prf> Thus, the base $AC$ (is) also equal to $DB$, and triangle $ABC$ is equal to triangle $BCD$ [Prop.~1.4].Thus, the diagonal $BC$ cuts the parallelogram $ACDB$ in half.