This entry covers the simple approach of aligning two objects along a reference side. This can play an important role when building a mesh that will be 3D printed and you want to make sure the bottom layer is perfectly flat and aligned.
The short explanation is to use the Align Objects command and set the Align Mode and axis.
For this example, I was making a keychain with a loop for a connecting. I made two objects, but I needed their bottoms to be aligned perfectly.
So I [Ctrl+Click] both objects, and then hit [Enter] and entered the Align Objects command.
In the bottom left Tools panel, there are some Align Objects parameters. For this case, I used the Negative Sides mode, and want to align along the Z-axis. Note, if this doesn't produce the effect you are expecting, sometimes it takes two attempts for the action to take.
Now from the side view, it is clear they are aligned along their bottom edges.
When brought into the 3D printer Slicer software, it is clear that the first layer to be printed was continuous, and didn't have any gaps or odd angles due to misalignment.