Click the Arrange | Order Objects | Move to Back command, click the button, right-click on an object and select Arrange | Move to Back, or press SHIFT+PAGE DOWN on the keyboard to move the selected object to the back. The object will appear behind all of the other objects in the layer.
In the left drawing, the red square is located in front of all the other objects. Use Move to Back to move the square to the back so that it appears behind of all the other objects (right drawing). Move Backward also moves the square back one object at a time.
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