Abstract The southern border of Baltica was for many years drawn along the NW part of the Tornquist Zone. This important boundary between segments of the pre-Cadomian East European and the Palaeozoic West European Platforms is now redefined on the basis of geophysical and well data. Most probably it runs south of the Ringkobing-Fyn High in Denmark. The transects Bornholm-Rugen and Scania-Holstein are used to discuss its position and tectonic character. Two main interpretations are possible: In model (1), the boundary coincides with a steep fracture zone (the Jylland-Mon Fault) that forms the northern front of the Caledonian folded Lower Palaeozoic in the southern forefield of Baltica. Because of significant differences in the tectonical, lithological and thickness development on both sides of this fault, it is interpreted as the northern boundary of a Caledonian terrane (Rugen-Pomorze Terrane) accreted to and locally overthrusted onto Baltica at Early Devonian times. The south adjoining area between this terrane and the northern front of the mid-European Variscides is thought to represent a mosaic of predominantly Cadomian terranes, probably with intervening Caledonian folded complexes. This mosaic of terranes is assumed to have been amalgamated to a North German Superterrane during late Caledonian diastrophism at the latest. In model (2), the boundary is ∼50 km south of the Caledonian front. It coincides with the buried Trans-European Fault that has been documented by deep seismic sounding. In this model, the Caledonian-deformed complexes south of the Ringkobing-Fyn High are interpreted as nappe structures of a Lower Palaeozoic marginal belt thrusted onto the southern edge of Baltica. Hence it follows that the area to the north of the mid-European Variscides is most probably built up of the internal parts of a North German-Polish Caledonide orogen.