The purpose for a center line on parts is to give the builder a visual marking to ensure the proper edge distance is achieved when attaching two or more parts together. It also allows the builder to line up parts with the pilot holes in the skins for drilling.
There are different methods for drawing the center line on a part. We recommend making the mark with a fine point Sharpie marker. Any color other than black will do (black can look like a shadow). Use your finger as a guide and draw a line along the center of the part. Another method is to lay the part flat on a bench and draw the line by running the marker around the edge. Here is a short video showing the process: http://eaavideo.org/video.aspx?v=1873835379
