In positioning an x‐ray tube in an experiment in order to gain optimum usable power, the variables at the operator's control are the take‐off angle and the collimation (or slit) system. These variables should be fixed by consideration of sample size, focal‐spot size, target ``absorption shadow,'' and the amount of divergence to be permitted. Consideration of these factors leads to an uncommon collimator for simple large single‐crystal diffraction work.