﹩\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).