Observed distribution of stellar Mt. Wilson S-indices over B − V, together with the location of basal chromospheric Ca II flux (see text). Although exhibiting the same basal flux, giants (shaded dots) show a systematic offset in S against main-sequence stars (black dots) because their photospheric spectral properties are gravity-sensitive (dashed and solid line, respectively). Most notably, Mt. Wilson Maunder Minimum-type stars and the plage-free solar S-values coincide with the Ca II basal flux limit.