Recent boundary layer observations complemented with results and ideas of many workers are synthesized into a geometrically explicit model of the magnetosphere in which the plasma sheet derives from the boundary layer on closed field lines at the flanks. The closed boundary layer, distinct from the mantle on open (or infinitely long) tail lobe field lines, extends down the length of the tail between nearly parallel and widely separated extensions of the dayside clefts. The plasma sheet is topologically bound by the volume of closed field lines. The extensive exposure of this volume along the flanks results in the well-known butterfly shape of the plasma sheet cross section. Closed boundary layer plasma fills flux tubes from the flanks, where it contacts the magnetosheath, to the ionosphere. The interior portions of these flux tubes lie between the plasma sheet and tail lobes and complete a circumscribing boundary layer plasma border to the plasma sheet. If dayside magnetic merging occurs, the dayside closed boundary layer is overlain by the plasma hood, which is the equatorward extension from the mantle of the region of open field lines in the usual merging picture. Along the flanks much of the closed boundary layer remains exposed to the magnetosheath. Observations consistent with the model are discussed.