Ratio r of the extrema of the mean longitudinal magnetic field (see text) against rotation period. Dots: stars with known rotation periods; triangles: stars for which only the lower limit of the period is known. Open symbols are used to distinguish those stars for which existing measurements do not cover the whole rotation cycle. Stars with representative points below the long-dashed horizontal line have reversing mean longitudinal fields. The two vertical (dashed) lines emphasise the tendency for longitudinal fields showing sign reversals to be restricted to rotation periods shorter than P_rot = 100 d or longer than P_rot = 1000 d.