We present a new technique for controlling the orientation of single-wall carbon nanotubes grown by chemical vapour deposition. The technique involves prior patterning of alumina islands on oxidized silicon substrates. Nanotubes growing on the oxide near an alumina boundary are oriented perpendicular to the boundary. Field-effect transistors were successfully fabricated from nanotubes produced using this technique. We argue that the orientation is caused by an electric field due to space charge associated with the alumina.