Free 3D Modeling Software - 3D Printing: Build your Own 3D Printer and Print your own 3D Objects - page 124

Figure 6.25 One final alignment is needed to stack the objects perfectly.
The result is shown in Figure 6.26 . I've rotated the camera view so you can see
that the two stacked objects are indeed lined up perfectly on the Z axis (up/down).
The camera view is looking up from underneath the flat workspace; imagine the
workspace as a glass table surface that you can see through, and you'll get the idea.