Radius–T_eff diagram for NGC 6791 and NGC 6819. The lines indicate the best-fit isochrones and the shaded regions indicate errors on the best-fit isochrones. The triangular points are the parallactic radius before correcting random errors using the cluster parallaxes. The circular points are the parallactic radii after correcting for random errors using the cluster parallax. See Section 2.4.3 for the parameters of the isochrones.