Naturally occurring error elds in tokamaks, which arise from misalignments of the external eld coils, may trigger the onset of tearing modes. The conditions under which a static error eld is able to lock a rotating tearing mode and how this process takes place are discussed. The analysis presented contributes to a new understanding and interpretation of mode locking, given in terms of the superposition of a slipping layer (a radial layer of very fast mode phase variations) and the tearing layer, where reconnection takes place. In addition, a stabilizing operating window is found to exist, independent of the phase time evolution of the mode.