  A new model for the substructure of quarks, leptons and weak gauge bosons is
discussed. It is based on three fundamental and absolutely stable spin-1/2
preons. Its preon flavour SU(3) symmetry leads to a prediction of nine quarks,
nine leptons and nine heavy vector bosons. One of the quarks has charge
$-4e/3$, and is speculated to be the top quark (whose charge has not been
measured). The flavour symmetry leads to three conserved lepton numbers in all
known weak processes, except for some neutrinos, which might either oscillate
or decay. There is also a (Cabibbo) mixing of the $d$ and $s$ quarks due to an
internal preon-antipreon annihilation channel. An identical channel exists
inside the composite $Z^0$, leading to a relation between the Cabibbo and
Weinberg mixing angles.
