  A new continuous classification system for BL Lacertae (BL Lac) objects is
proposed. The peak frequency of the synchrotron component in the $\nu L_{\nu}$
spectrum is used as a classifying parameter. The application of the
accelerating inner jet model to complete BL Lac samples suggests that the range
of the observed properties may be explained by adopting a description based on
two parameters: the jet angle to the line of sight, and the relativistic
electron kinetic luminosity.
