I spent an hour perfectly measuring the position of 3 nails to hang 3 identical picture frames on my wall. Everything went fine and I'm certain that from a mathematical point of view the nails are exactly in the right positions.

But once I hang the picture frames on them, it looks totally crooked. They all lean towards one side and the edges don't line up. Maybe the frames are really a bit heavier on one side, or my wall isn't really smooth, but every time I nudge them into level, they just move back into their crooked positions.

How can I level a picture frame that hangs from one single nail?

I won't accept solutions that:

• require driving in any more nails
• are visible in front of the frames
• leave permanent marks on the wall / wallpaper, like some glues and tapes do

This is the back of a frame and I hang them from the little hanger marked in green. The hangers are positioned perfectly at the center.