Center line on this rectangular bar is drawn with the help of parallelogram. 

Normally we will use a ruler to mark center points at a couple of locations. Joining these two points will give us a center line. We can instead use parallelogram for the same. These four 3D printed bars are the same size with holes at equal distances. We will use M3 screws to fix these in place. Now we can move these bars and our parallelogram is ready. But this can not be used as is for our purpose. 

Use M3 screws to fit everything in place.

Explainer video for the same…