Send Backward
The Layout | Move | Backward command or the 
button move the selected object backward one position. Alternatively, right-click the object and select Order Objects | Move Backward.
Objects can also be moved by dragging them to a new position in the Contents window.
The default keyboard shortcut for the Layout | Move | Backward command is CTRL + PAGE DOWN. The keyboard shortcuts can be customized with the Customize command.
Example
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In the left drawing, the red square is located in front of all the other objects. Use Send Backward to move the square back one layer so that it appears between the circle and the triangle (middle drawing). Clicking Send Backward again places the square on behind all of the other objects (right drawing). Send to Back places the square on behind the other objects.