Truck Camera 
Activate a Camera viewport. > Viewport navigation controls > Truck Camera
Truck Camera moves the camera parallel to the view plane.
For a target camera, dragging the mouse moves both the camera and its target parallel to the Camera view.
This button replaces the Pan button when a Camera viewport is active.
To truck a camera:
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Activate a Camera viewport.
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Click Truck Camera.
The button highlights when it is on.
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Drag to move the camera and its target.
The camera and its target move parallel to the view plane, which is perpendicular to the camera's line of sight.
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Press Esc or right-click to turn off the button.
To constrain trucking to a single axis: