In Figure 11 let ABCD be a contraparallelogram having AB = CD, AD = BC.
Let O, P, Q, R be the midpoints of AB, BC, AD, DC respectively. They obviously lie in the line parallel to AC and to BD, and equidistant from them.
Let U be the mid-point of AC and V that of BD.
OURV is a rhombus each of whose sides is half of AD or BC, and parallel to AD or BC. PUQV is a rhombus each of whose sides is half of AB or CD, and parallel to AB or CD.