Dear Diman,

lines appear if your sample is oriented so that there is only one set of crystallographic planes (e.g. 100) parallel with the beam (i.e. perpendicular to the image plane).

If you tilt the sample so that two sets of planes (e.g. 100 and 010) are parallel with the beam, you get two differently oriented set of lines in your image forming spots on their intersections. The two sets of planes parallel with the beam allow the atoms to get aligned in columns parallel with beam (i.e. perpendicular to the image plane and parallel to direction 001 in our example). That's what forms the spots in the image.

Hope it helps.

Best wishes,

Mila

Thank you, Dear Mila,

your answer helped me a lot.