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Operation Toolpad > Mesh Command Button 
> Edge Command Button 
> Mesh Edges 
Shift-click or bring up the Edge List window and select both the vertical lines. If this is difficult, one can zoom in on an edge by holding down the Ctrl button, clicking and dragging the mouse to specify an area to zoom in on, and releasing the Ctrl button. To return to the main view, click on the Global Control Toolpad > Fit to Window Button again. You can also hold down Ctrl and double-click in the window to zoom out to a fitting window. To pan the view, hold down the middle mouse button and drag the mouse.
Once a vertical edge has been selected, select Interval Count from the drop down box that says Interval Size in the Mesh Edges Window. Then, in the box to the left of this combo box, enter 5 for the interval count.
Click Apply. Nodes appear on the edges showing that they are divided into 5.
(Click picture here for larger image )
Repeat the same process for the horizontal edges, but with an interval count of 100.
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Operation Toolpad > Mesh Command Button > Face Command Button 
> Mesh Faces 
Shift left-click on the face or use the up arrow next to Faces to select the face. Click Apply.
(Click picture for larger image)
Go to Step 3: Specify Boundary Types in GAMBIT