Source compactness (SC, top panel) and core dominance (CD, bottom panel) distributions for the present sample of BL Lacs using the classification given in Sect. 4.3. U.L. indicates that the corresponding values are upper limits. Gamma-ray BL Lacs are reported with black dots. In the CD histogram, we plot sources (all radio galaxies, but 2 BL Lacs) of the Bologna Complete Sample (Liuzzo et al. 2009). Two different symbols are used to represent the three gamma-ray BCS sources: M 87 (star) and 2 BL Lacs (square). Arrows indicate upper- and lower-limit CD values.