The authors use the PQEq force field based on quantum mechanics studies to provide a first-principles description of the 90° domain wall in barium titanate. Using periodic cells with lengths of up to 74nm (5120 atoms), the authors find that the domain wall thickness is 21nm in which the polarization switches over a 5nm central layer surrounded by two transition layers each 8nm wide. The central 5nm layer consists of a 2nm sublayer with overshooting polarizations and a 3nm sublayer with reversed polarization. This structure explains the discrepancies in previous analyses, which have suggested domain walls ranging from 100nm to 1nm. These results should be of particular importance in developing nanoscale ferroelectric devices.