It is shown how the constant opening angle of the Centaurus A jet can arise from a pressure-confined fluid jet of non-circular cross-section. The slow decrease in surface brightness is predicted even for a pure adiabatic expansion. Evidence for flattening in other jets is discussed. Internal shocks will produce linear structures such as observed in the M87 jet. Magnetic pressure will eventually dominate, resulting in focusing and a return to a circular cross-section. Conversely, shocks may be the catalyst for further flattening.