We use the rotational discontinuity-slow expansion fan model for an open magnetotail boundary to obtain a quantitative three-dimensional picture of the complete magnetotail boundary. Its configuration and physical properties are inferred for different orientations of the field as well as different reconnection rates by representing the high-latitude plasma mantle with a self-similar slow expansion wave. Some of those properties follow: (1) The tail boundary geometry appears to be stable against moderate variations of the upstream parameters. (2) The transition between the open and closed portions of the tail boundary takes place at increasingly higher latitudes tailward, thus narrowing the open window in the same direction. For the magnetosheath values considered (n0=107 m-3, V0=3¿105m s-1, B0=10 nT, T=106 ¿K) and for a purely southward field an initial 90¿ latitudinal width of the open window in the near-Earth environment evolves into 55¿ at x≂-150 RE. (3) Portions of the plasma mantle become separated from the magnetosheath by a tangenial discontinuity as larger distances down the tail considered, with a thin strip of plasma sheet plasma (≂2RE in the radial direction, at x≂-150 RE) intruding in between. (4) The internal boundary of the mantle is relatively flat in the near-Earth tail but becomes increasingly V shaped tailward. Its intersection with the geomagnetic equator conforms to a U-shaped form with an antiearthward concavity. The tail boundary geometry when the external field has some inclination away from the vertical is investigated. A duskward or dawnward shift of the entire open tail boundary takes place, and the expansion fan is thickest on the sector toward which the shift occurred. ¿ American Geophysical Union 1990