﹩\mathrm{log}\langle {R}_{\mathrm{HK}}^{{\prime} }\rangle ﹩ vs. ﹩\mathrm{log}(\tau /{P}_{\mathrm{rot}})﹩ for the stars of M67 with known rotation periods as green uppercase letters, the F and G dwarfs of BMM as blue italics characters, the K dwarfs of BMM as red roman characters, and the four stars of SB with ﹩{P}_{\mathrm{rot}}/\tau \geqslant 2.4﹩ as orange numbers 1–4. On the upper abscissa, the Rossby number ﹩{P}_{\mathrm{rot}}/\tau ﹩ is given. The dashed–dotted line shows the fit of BMM, whereas the solid line represents a fit to the residuals in Equation (5) for the nine stars with ﹩\mathrm{log}\langle {R}_{\mathrm{HK}}^{{\prime} }\rangle \geqslant -4.85﹩. The dashed line is a direct fit to the same nine stars and the dotted line shows the fit given by Equation (6). The arrow indicates the anticipated evolution with increasing age *t*. Some of the symbols have been shifted slightly to avoid overlap. The Sun corresponds to the blue italics *a*. The upper inset shows the residual ﹩\mathrm{log}c﹩ vs. ﹩\mathrm{log}\langle {R}_{\mathrm{HK}}^{{\prime} }\rangle ﹩ for the stars of M67 as green filled circles, the F and G dwarfs of BMM as blue diamonds, and the K dwarfs of BMM as red crosses. The lower inset shows the increasing magnetic field strength for small values of ﹩4\pi \tau /{P}_{\mathrm{rot}}﹩ from Figure 12(b) of Karak et al. (2015).