I am no damn expert, but I've used them. If you're actually going to use it: Choose a size that you can read and fits in your hands Choose a smooth slidi

I am no damn expert, but I've used them. If you're actually going to use it: Choose a size that you can read and fits in your hands Choose a smooth sliding Indicator (some have magnifiers) and Slider without slop Choose scales you'll use (typ 2 linear and 2 log, some have trig roots etc) Choose one with the precision you think you'll need I think most of them are plastic, but some feel better in the hands. There are of course metal, wood, and I think ivory. There are cylindrical ones, and all sorts of historical examples. If you're going to carry it around you'll want a case (or a pocket protector) to put it in.