Sketch of the disk and magnetic field configuration for an accreting system (not to scale). The truncation radius R_t indicates where the matter, indicated by the mass accretion rate Ṁ, couples to the magnetic field. The co-rotation radius where the magnetic field lines rotate at the same speed as the local Keplerian speed is at R_co. The outer radius, R_out, indicates the outermost radius where the central magnetic field is connected to the disk. Beyond this radius the stress becomes too large, causing the field to disconnect from the central source.