iDR2 recommended parameters of the stars of the calibration clusters in the T_eff − log  g plane. No attempt was made to identify non member stars, i.e., the plots include all stars observed in the field of the clusters. Ages and metallicities were taken from the catalog of Harris (1996, and online updates) for the globular clusters and from the WEBDA database for the open clusters. The isochrones were computed with the web-tool of the PARSEC group (Bressan et al. 2012, all with solar-scaled composition), solid lines, and with BeSPP (Bellaterra Stellar Parameter Pipeline, Serenelli et al. 2013, α-enhanced below [Fe/H] = −0.80) which uses the GARSTEC stellar evolution code (Weiss & Schlattl 2008), dashed lines. Error bars represent the method-to-method dispersion of each atmospheric parameter (see Sect. 7.3).