Purpose: describe what a paned window is, and where to find some implementations of one.
A paned window is two windows with a bar down the middle, where the bar has a handle for resizing the two windows.
BWidgets [L1 ] has a PanedWindow.
ClassyTk [L2 ] has a Paned widget.
obTcl [L3 ] has a paged window widget.
Jeffrey Hobbs' widget package [L4 ] includes a pure-Tcl geometry-manager-like-thing that serves as a paned window.
Iwidgets [L5 ] (based on itk) includes a panedwindow.
TIP 41 [L7 ] proposes a built-in PanedWindow.
[Does Perl/Tk have a Pane widget?]