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When dropping controls from the palette on an empty canvas or above or below an existing control, the newly dropped control is 12 columns wide. The trigger control is an exception. 
If you drag a control from the palette and drop it to the left or right of a control, the newly dropped control will take the width of the control you dropped on. The width of a control that is already in your form must be adjusted manually by the designer if the position of the control is changed.
This approach makes It very easy to create a form with a muti-column layout. There are two ways to accomplish this:
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Watch this video for a demonstration.
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Page Break controls have special rules for drag/drop. Click here for more information.