TixPanedWindow
Creating a PanedWindow and its panes:
tixPanedWindow .p -orientation horizontal
set p1 [.p add left -expand 1 -size 200]
set p2 [.p add right -expand 4]
tixDirTree $p1.d; pack $p1.d -expand yes -fill both
tixScrolledTList $p2.t; pack $p2.t -expand yes -fill both