Normalized Stokes parameters (q_obs, u_obs) of Cyg X-1 in the V band. The red circle with the error bar is the average polarization of Cyg X-1 in 2022. The crosses of different colors (from cool to warm) with error bars correspond to the average polarization in each of the 11 seasons of PMO observations in 1975–1987. The dashed blue and solid red lines show the directions of the average intrinsic polarization vector of Cyg X-1 for PMO and our observations, respectively. The other symbols correspond to the data obtained in other epochs, as described in the inset.