Relevant to Blender v2.31
To make an object rotate so that it faces another object, and keep this facing even if either object is moved, select at least two objects and press CTRL-T. A dialog appears asking if you want to use a Track constraint or the old (Pre-2.30) track system. The Track constraint will be analyzed in the section called “Constraints” and is the preferred method.
Here we will briefly treat the old track system, so, let's assume you have
Old Track in the dialog.
By default the inactive object(s) now track the active object
so that their local y axis points to the tracked object.
However, this may not happen if the tracking object already has a rotation
of its own. You can produce correct tracking
by canceling the rotation (ALT-R) of
the tracking object.
The orientation of the tracking object is also set so that the
z axis is upward. To change this, select the tracking
object, change the Button Window to Object Context (
F7) and select the track axis from the first row
of six radio buttons and the upward-pointing axis from the second in the
Anim Setting panel.
(Figure 5.5, “Setting track axis.”).
To clear a track constraint, select the tracking object and press ALT-T. As with clearing a parent constraint, you must choose whether to lose or save the rotation imposed by the tracking.