The self-thinning rule describes an outer boundary on the trajectories which even-aged stands of plants can follow in a space of log biomass v. log density. An important variant to the rule occurs in dense shade, but the exact nature of the variant remains unclear. Three alternative models of the effect of dense shade on the rule are described. Stands of Trifolium subterraneum were grown in a glasshouse in full daylight and others shaded at 30% of full daylight; after a period, some full-light stands were moved to the shade, and some shaded stands to full daylight. The slope of the trajectory of a stand growing in shade is determined not only by the light climate, but also by its position with respect to the relevant thinning line. This observation excludes one of the models.