Free 3D Modeling Software - 3D Printing: Build your Own 3D Printer and Print your own 3D Objects - page 123

Figure 6.24 Not centered along the other axis.
As you can see, they may be centered when looking from the front, but they are
not centered from the side. But that's an easy fix. Select both objects, click the Ad-
just menu, and select Align once more. Click the dot indicated in Figure 6.25 to
center the two stacked objects along this center line.