  We present an antenna shower formalism including contributions from
initial-state partons and corresponding backwards evolution. We give a set of
phase-space maps and antenna functions for massless partons which define a
complete shower formalism suitable for computing observables with hadronic
initial states. We focus on the initial-state components: initial-initial and
initial-final antenna configurations. The formalism includes comprehensive
possibilities for uncertainty estimates. We report on some preliminary results
obtained with an implementation in the Vincia antenna-shower framework.
