Blazars have two subclasses, namely, BL Lacertae objects and flat-spectrum radio quasars. The two subclasses share many similar observational properties, but their difference in emission lines cannot be ignored. What causes the difference? In this Letter, we propose a method to determine the intrinsic ratio, f, of the comoving luminosity in the jet to the unbeamed luminosity and find that f in BL Lacertae objects is greater than that in flat-spectrum radio quasars. Therefore, the emission from the jet will dominate the unbeamed (line emission) part in BL Lacertae objects. However, in the case of flat-spectrum radio quasars, the emissions from the jet are comparable to the unbeamed part. We propose that the difference in f perhaps plays an important role in the different emission-line properties of BL Lacertae objects and flat-spectrum radio quasars and that there is no evolution effect between the two subclasses of blazars.