A statistical analysis of all the IMP 8 tail boundary crossings between 1974 and 1982 has been carried out. Minimum variance analyses of magnetic field data are combined with plasma data from the Los Alamos National Laboratory plasma analyzer to determine the nature of the magnetopause crossings and whether there is a plasma mantle. It is found that the open tail boundary is to a great extent appropriately described by a rotational discontinuity-slow expansion fan. The properties of this boundary are investigated and evidence of a noticeable duskward (dawnward) tilt of the symmetry axis of the open portion of the tail boundary (≂24¿) with respect to the north-south line is found for an eastward (westward) interplanetary magnetic field (IMF) orientation. The average size of the open portion of the boundary in that case is estimated at ≂110¿. The average normal magnetic field component normalized to the total field magnitude, Bn/B, for an east-west IMF (≂0.182) yields a cross-tail EMF more than twice that estimated for the typical cross-polar cap if the open portion of the tail boundary is assumed to consist entirely of a rotational discontinuity-slow expansion fan. There is evidence, however, that the open portion of the tail boundary may have a structure of alternated open and closed portions in a geometry where narrow, tail-aligned tangential discontinuity strips intrude between strips of rotational discontinuity surfaces. The average cross-tail potential drop in that geometry is estimated to be ≂20% smaller than the potential drop across a tail boundary composed entirely of a rotational discontinuity-slow expansion fan. ¿ American Geophysical Union 1990