(a, left) Geometry of the emission region, which moves with a velocity β, as seen in the source frame. Synchrotron radiation (electric field e_obs) is emitted by relativistic electrons which move in a static magnetic field B_obs. The vector e_obs (B_obs) makes an angle ξ (η) with the plane containing β and the observer direction n, and its projection on this plane makes an angle θ (ψ) with the z axis. Reproduced from Blandford & Königl (1979). (b, right) Observed polarization angle, ξ, measured clockwise from the projected jet axis computed for a jet with a bulk Lorentz factor of Γ = 10, viewing angle of 5.7° (i.e., roughly the critical angle 1/Γ), and threaded by a helical magnetic field with a pitch angle of α_b = 89° (i.e., mostly toroidal). The time evolution of ξ is computed considering a K-H instability with a projected wavelength of 100 μas, and two perturbations moving at apparent velocities of 10.2 c and 17.4 c, to resemble observed components P1 and P2. The observed polarization angles of components P1 and P2, measured with respect to the jet axis, are also shown.