There are three generations of fermions, but the relation between them remains a mystery. Modern particle theory relies heavily on symmetries, so it seems very abnormal if there is no (horizontal) symmetry between the three generations as well. In recent years, there are many attempts to explain the neutrino-mixing pattern by a horizontal symmetry, but unfortunately none of them are very compelling. In any case, the resulting symmetries are very different from those of the quarks, which is disturbing in light of the similarities between leptons and quarks in many other ways. I will show that if leptons and quarks are unified according to SO(10), then there is a common horizontal symmetry for them. Moreover, this horizontal symmetry is compelling because it follows from the (vertical) symmetry SO(10).