The replication of fermion generations is one of the outstanding puzzles in particle physics. Could there be also a fourth generation of quarks and leptons ? There is no convincing theoretical or experimental reason why not. In some grand unified schemes consistency with the evolution of masses and couplings may indicate only three generations, but these models are not uniquely established. Thus it is important for collider experiments to continue the search [...]
We label [...] the fourth generation quarks by (a,v) where a and v have charges (2/3, -1/3). This notation for the quarks is suggested by the alphabetical labels used for the other quarks (viz. a,b,c,d,....,s,t,u,v). The names audio and video have been proposed for these quarks, after the gods of the present generation!

V. Barger&R. Phillips, "Collider Physics", Addison-Wesley (1987) p. 467.