Purpose of tracking detectors with "stereo design" HERA-B used silicon microstrip detectors for their vertex detector. Behind each two sided detector with strips in x- and y direction, a second identical detector was placed, rotated by 5 degrees. LHCb uses a similar design for the phi-detector in their vertex locator. I don't understand the purpose of this geometry. Of course an additional detector in a different position increases resolution, but why exactly 5 degrees and not, say, 45 degrees? How is that "stereo"? I read in an LHCb doc: "This ensures that adjacent stations are able to distinguish ghost hits from true hits through the use of traditional stereo view." But this still doesn't explain the rotation angle. Couldn't this distinction be achieved without any rotation?