Context. Blazars are usually classified following their synchrotron peak frequency (nuF(nu) scale) as high, intermediate, low frequency peaked BL Lacs (HBLs, IBLs, LBLs), and flat spectrum radio quasars (FSRQs), or, according to their radio morphology at large scale, FR I or FR II. However, the diversity of blazars is such that these classes seem insufficient to chart the specific properties of each source.