How can I align two faces in edit mode I have two triangular faces that are connected with one edge (one object, in edit mode). I would like to move the second triangle into the plane of the first triangular face. This means that the third not connected point will have to move.

This would be easy if the plane would be x,y or z by using the scale trick, but I would like for it to work in any given plane. How could I do this?
I think the answer is: yes you can.


Select the connecting edge and snap the 3D cursor to it
Set the pivot point to 3D Cursor
The connecting edge still selected, add a custom transformation orientation and keep it as orientation


Now


Select the first triangle (the target)
Use ShiftNumpad 1 or ShiftNumpad 3 to be in orthographic view aligned with this triangle and so that the connecting edge is aligned to the view. This is useful in order to snap as we want later.


To snap:


Enable snapping, set it to vertex, snap with closest, project onto self and enable it for rotation


To rotate:


Select the opposite vertex (the one of the other triangle)
Rotate R, hit Y to rotate around the connecting edge
Set the mouse cursor onto the opposite vertex of the target triangle


To rotate to the opposite:


Rotate R, hit Y, then hit 180 on the numpad