This is a parametric mount for attaching a cylindrical flashlight to those sliding handlebar grips that come with many bike lights (such as this one: http://www.tesco.com/direct/via-velo-white-led-mini-front-light/729-9776.prd?pageLevel=&skuId=729-9776). It's useful if (like me) the slide on your existing bike light has split, and you have a good flashlight/torch which you'd like to use as a bike light.The first print came out slightly small, so I had to file both it and the rail down, but that may have been a printer miscalibration error. After that it's worked fine (at least so far), so I'm putting it out to the world in its current state.