How can I snap to the projection of a mesh element? I am trying to model a car seat. I would like to be able to Grab the selected edge (marked with the red arrow) and translate it to an arbitrary location. However, I would like to constrain the median point of the edge to the plane that goes through the middle of the volume below it, on the Z axis, parallel to its (large) faces, i.e. not move the edge "outside" the volume.

Unfortunately I cannot use regular constraining, because the volume in question is rotated from both the Global and the object Local axes. Also, the normal space of the edge is not aligned to the normal space of the volume.

I am thinking I can do what I want either by (a) constraining the movement to the YZ plane, in the normal coordinates of one of the large faces of the volume below, or (b) by snapping the vertices of the edge being moved to the projection along the Z axis of the mesh geometry.

How can I do this? Is there any other (blender-preferred) way to achieve this?
You can achieve this by aligning the view to the desired plane with Shift Numpad 7 and then constraining the movement to view.

To constrain movement to the YZ plane, you can press ShiftX (where X is the axis to not allow movement on) after pressing G. See Scaling a cylinder along both X and Y axis.

You can set the transform orientation  in 3D view > Header:




Local uses the orientation of the object.
Normal uses the normals of the selection.
View Uses the orientation of the view.


Note that you must press the axis constraining keys twice in order to use the selected orientation, otherwise global is used. E.g. GShiftXShiftX.

For using the View orientation, some useful tools/techniques are:


ShiftNumpad 7 will align the view to the normal of the selection (Numpad 1 and Numpad 3 will align the view to the side of the normal). This can be useful to use the normal of one part of a model to translate another part.
Using a separate 3D view for orientation and transforming, and viewing the model in a perspective 3D view from a better angle.