  Mass spectra of baryons consisting of two heavy (b or c) and one light quarks
are calculated in the framework of the relativistic quark model. The light
quark-heavy diquark structure of the baryon is assumed. Under this assumption
the ground and excited states of both the diquark and quark-diquark bound
system are considered. The quark-diquark potential is constructed. The light
quark is treated completely relativistically, while the expansion in the
inverse heavy quark mass is used revealing the close similarity with the mass
spectra of B and D mesons. We find that the relativistic treatment of the light
quark plays an important role. The level inversion of the p-wave excitations of
the light quark in doubly heavy baryons is discussed.
