Misaligned dropouts can lead to all sorts of problems: poor tracking, bad shifting, excessive wear on hubs, and more. These gauges install into both fork and frame dropouts to provide a means to check dropout alignment, and in some cases correct it.
Steel frames can, within reason, have their dropout alignment corrected with these tools. It's best to consult with your frame manufacturer if you've got a frame made of another material.
Use this on the rear dropouts in conjunction with our Hanger Genie to ensure perfect shifting.