_NET_SYSTEM_TRAY_ORIENTATION orientation CARDINAL/32

  #define _NET_SYSTEM_TRAY_ORIENTATION_HORZ 0
  #define _NET_SYSTEM_TRAY_ORIENTATION_VERT 1
      

The property should be set by the tray manager to indicate the current orientation of the tray. Tray icons may use this hint in order to maintain the icon's aspect ratio and also as an indication of how the icon contents should be laid out.