Procedure used to estimate the fraction of BSSs in NGC 188. Cluster and reference-field stars are marked with gray circles and brown crosses, respectively. The gray dashed line represents the ZAMS. The gray solid line is the fiducial line of the MS and the SGB shifted to the blue by four times the observational color error (σ). The fiducial line of MS-MS and SGB-SGB binaries, shifted by four times σ to the red is represented by the red line. Region C of the CMD is indicated as an azure-shaded area, while region D, marked with the gray shade, is enclosed by gray and red solid lines and by the two horizontal black dashed lines. Cluster field stars in regions C and D of the CMD are marked with azure solid and black-shaded circles, respectively.