A differential pair is any two transmission lines. When each transmission line has a return plane, its pretty clear how the differential impedance is related to the geometry. But what if there is no return plane? How do we think about the differential impedance of a differential pair when we remove the return plane, like in a twisted pair? What quality influences the differential impedance and how do we go about measuring the differential impedance? What is really different between two transmission lines with a return plane and a twisted pair with no return plane? We will explore these questions using essential principles, 2D field solvers and TDR measurements.