The TabStripPlacement property is inherited from the TabControl and it is not supported by the Docking control - it is not implemented in the RadPaneGroup's source code so it doesn't work with the control.
However, in order to achieve the desired appearance you can modify the default Template of RadDocking. Please, check the attached sample that demonstrates what modification need to be made in the LeftTemplate of the PaneGroup in order the Panes to be visualized left as expected.
Hope this helps.
Telerik by Progress
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