Use Unior's Derailleur Hanger Alignment Tool to ensure your rear derailleur hanger is paralleled with the rear wheel, and if it's not, to make alignment adjustments. Unior has been making tools since the better part of the last century, and it makes its tools out of durable materials that are designed to last — in this case, that means steel. If you're still getting the hang of at-home wrenching, Unior also includes complete instructions for making adjustments so you can be sure you're on the right track.
Old school and good at what it does
Familiarity: I've used it once or twice and have initial impressions
Now days with replacable deraileur hangers this isnt such a common tool to use, but we have a few nice steel frames in our stable. A properly aligned rear hanger provides perfect shifting and this is the perfect tool for the job. Nice solid feel and excellent control. Just the right resistance at the gauge.