The Puzzle

I have resolved a strucutre with space group of P21, with cell angles: 90.0 99.50 90.00. The cell content analysis has shown that there are two units in an asym unit. Actually, this structure is quite a simple one, I have found the correct resolution by molecular replacement after searching for one unit model.
When I have the self rotation function, however, one thing puzzled me a lot. Why there are two 2-fold peaks?

The Answer

The reason is similar to ‘Why there is no P22 space group’, a 2-fold screw axis has been interacted with the 2-fold axis of the asym unit. First step, find the 2-fold screw axis vertical to the screen. And then, you can find the generated 2-fold screw axis from the side. This is why there are two 2-fold peaks.