  We consider union-closed set systems with infinite breadth, focusing on three
particular configurations ${\mathcal T}_{\rm max}(E)$, ${\mathcal T}_{\rm
min}(E)$ and ${\mathcal T}_{\rm ort}(E)$. We show that these three
configurations are not isolated examples; in any given union-closed set system
of infinite breadth, at least one of these three configurations will occur as a
subprojection. This characterizes those union-closed set systems which have
infinite breadth, and is the first general structural result for such set
systems.
