Left panel: upper thumbnail shows an AR in high chromosphere (center of Ca II K3 line), while the lower one shows the same region at the same time, in low chromosphere (Hα line). Right panel: same active region, in low corona, on 2001-07-03 (upper image) and on 2001-07-04 (lower image). Left observations were realized with the Meudon spectroheliograph and are available in BASS2000. Right observations come from the SOHO/EIT instrument and are available at NASA. Automatic detections come from Barra et al. (2009).