[Avenger1601]

Say that I've got a primitive cubic structure, and I've run XRD on it, and successfully identified reflections such as 100, 110, 111, 200 etc.

If the symmetry of my substance is then lowered to tetragonal, which of these reflections would I expect to see become split into doublets?

Some sources say that it's any case where I have only two sides equal, others say that it's whenever I don't have (hkl) = (xxx), i.e. h =/= k =/= l.

Looking for clarification, hoping you all can help.