Reference frames used for maser positions and velocities, and magnetic fields. (a) Transformation of coordinates by rotation around the line-of-sight axis los over the position angle of the jet to bring Dec onto the sky-projected jet axis xl. (b) Transformation of coordinates by rotation around the axis R over the inclination angle of the jet to bring xl onto the jet axis ɀ. (c) Transformation of coordinates by rotation around the axis ɀ over the rotation angle ϕ of the maser position to bring oɀ along the direction ê_ρ. (d) Decomposition of the maser velocity into the axial (v_ɀ), radial (v_ρ), and azimuthal (v_ϕ) components.