  We present a new minimal model for the substructure of all known quarks,
leptons and weak gauge bosons, based on only three fundamental and stable
spin-1/2 preons. As a consequence, we predict three new quarks, three new
leptons, and six new vector bosons. One of the new quarks has charge $-4e/3$.
The model explains the apparent conservation of three lepton numbers, as well
as the so-called Cabibbo-mixing of the $d$ and $s$ quarks, and predicts
electromagnetic decays or oscillations between the neutrinos $\bar{\nu}_{\mu}$
($\nu_{\mu}$) and $\nu_e$ ($\bar{\nu}_e$). Other neutrino oscillations, as well
as rarer quark mixings and CP violation can come about due to a small
quantum-mechanical mixing of two of the preons in the quark and lepton wave
functions.
