This encoder is made for the RepRap project, but it can be used for any project where you wish to determine angle or rotational speeds. The encoder on the extruder is an important step towards better 3D print quality (constant plastic flow rates).
It can be laser cut, used as a template for cutting, printed on an overhead/transparent sheet, etc. You can adjust the sizes, since it's SVG. It was made in the open source program called inkscape, which will work very well with this file.