This paper gives the first explicit test of the rotational discontinuity‐slow expansion fan structure predicted by MHD models of the open tail boundary. We analyzed the magnetic field and plasma data collected by IMP 8 during its magnetotail boundary crossings at geocentric distances near 25Reand high latitudes. Many crossings show the particle and field signatures of the paired MHD tail boundary structure. The openness of the tail is revealed by rotational discontinuities. They often show relatively large normal magnetic field components. Other crossings show properties that suggest a tangential discontinuity and reveal regions where or times when the tail is closed. We give examples of each her