a) SGB zoom-in on B vs. (B − K_S) CMD of NGC 6656. The four points (and the two straight lines) used to perform the linear transformation are plotted in red. b) Rectified SGB. The red horizontal lines are set at “Ordinate” 0 and 1; the gray solid line is set at Δ“Abscissa” = 0. c1) Dual-Gaussian fit in black; individual Gaussian in blue and red are used for bright and faint SGB respectively, for the SGB-star sample between 1.5 and 3.0 arcmin from the cluster center. c2) As for c1) but in the range 3.0−9.0 arcmin. c3), e), d) Same as in panels a), b), c1), c2) but for the HST data in the m_F275W vs. (m_F275W − m_F814W) plane.