Please could someone help with suggestions on how to make this prop: It's an old navigation tool called a cross-staff, consisting of a pole and a cross piece which is moved up and down the pole at right angles to the ground. (see attached image). I'm not sure how to make a prop of two parts where one part moves along the other (constrained to one axis). If anyone can help (and understands what I have written here!! :)) I would appreciate it very much. I intend to use it for a demonstration for a video display in an exhibit on Portuguese navigation at the museum I work in.